Let's start with a little math (and then move on to much more math). How many worlds are there in the Old Republic (by world, I mean populated mass, whether it's a planet, moon, or something else)? To answer that question, we look to the Galactic Senate. Remember those shots of the huge Senate room with the floating platforms? Each of those represents not just one world, but one system, which may have several worlds in it. How many worlds is that? Thousands. And that's just the Republic members. Even assuming most populated worlds are members of the Republic, that's a lot of populated worlds.
According to Wookiepedia, George Lucas once said there are 24,372 worlds in the Republic. Even taking that with the typical grain of salt that all of Lucas's statements that aren't directly in a movie come with, let's use that as a baseline. 24,372 worlds with at least enough population and development to qualify for membership in the Republic.
Now, let's come to Earth. We have a population of roughly six billion. Of those, perhaps two billion live in what we could call reasonable comfort (less than luxury but better than poverty). According to Wikipedia, the world has 17 million active duty troops. Even assuming a third of that (taking just the part of the world that lives okay, because Star Wars worlds generally seem to do all right for themselves) that's at least five million troops.
In other words, we have five million troops from a planet like ours. Admittedly, Earth is pretty well populated... until you consider that the planet is mostly ocean and our agricultural practices are pretty slim. Naturally, we'll have a higher population than Tatooine or Mustafar, but I assume Coruscant must balance worlds like that out.
Then again, according to Wookiepedia, Naboo (which seems outrageously fertile), only has a population of 600 million, making it one-tenth the Earth's population, and one-third the number I stated as a benchmark. But you repeatedly hear Naboo referred to as a small world, so I have to assume it's an exception. Ryloth is listed as having a population of 1.5 billion, and that's a pretty backwards agricultural planet. Corellia weighs in at 7 billion, so I seem to be pretty close.
So, let's assume that my previous numbers are sound. Two billion people per world, five million troops per world, and more than twenty-four thousand worlds. That comes to 120 billion troops.
Okay, so say that the vast majority of worlds in Star Wars aren't warlike or don't feel like having a standing army (and can't raise one in an emergency such as the Clone Wars, either, which is a stretch). So let's take that number by one-tenth, because I'm being generous. Twelve billion soldiers, being, as we've seen, a very conservative estimate for the amount of soldiers that we can call on.
Devaronian's advocate: Many worlds, if not most, are ill-suited to serve as regular soldiers. Several races in Star Wars (Ewoks, Toydarians, Tusken Raiders, etc.) could never be in a galactic army. Counter: Let's be honest: the majority of worlds in Star Wars are human or close enough to human; the rest are rare exceptions.
Devaronian's advocate: Many worlds, such as moons and colonies, are vastly underpopulated, therefore my average of two billion is too high. Counter: Come on. Coruscant has one trillion people living on it. That's 500 worlds right there. Besides, in order to have representation in the Galactic Senate, there's got to be at least some people living there. According to Wikipedia, Alderaan has about 2 billion people living on it (pre Death Star), and there's nothing to suggest it's exceptional in its population.
Next, let's add some more numbers to try to understand how many soldiers there would have been in the major battles of the Clone Wars. Total number of clones from Kamino? Wookiepedia lists three million clones as the general standing number. There's some disparity about numbers, but even assuming the very highest number they guess at, which is 700 million, that's still very slim.
How slim? Let's take those 24 thousand worlds again. That's about 30,000 clones per world. The podunk city I live in has a population of 42,000. Assuming we're particularly scrappy, we could take those clones by ourselves.
Let's look at it from another perspective. Remember that our planet has 17 million active duty troops? It also has about 45 million reserve troops. Assuming those would be called up if the planet was threatened, we'd be looking at about 1/11 of the Grand Army of the Republic right here on Earth.
How many major campaigns theaters are there in a war encompassing Earth? Half a dozen at least. And yet in each major planetary campaign, we see only one big battle. They're even referred to as battles: the Battle of Kashyyk, the Battle of Mygeeto. If you're really trying to conquer a planet, you would have to fight long campaigns over vast theaters, not conquer one big city and call it a day. And for that, you need many more troops.
Devaronian's advocate: Although the clones form the core of the Republic army, the Grand Army of the Republic is actually composed of local forces who fight within their own sector. Counter: In virtually every representation of the Clone Wars, we see clones, not local soldiers, fighting. The reason given for forming the Grand Army was so there wouldn't have to be an army drawn from the worlds; there was an attempt to create a separate army in the comics, but it failed, which suggests the clones form at least the majority of the Republic's army.
Remember the battle for Hoth? The Rebel Alliance was more or less quartered within a base the size of a standard military base. How many troops could possibly have been involved in that fight? Not even a million. Considering some of the numbers we've been bandying around (24 thousand worlds. Twelve billion troops.) that's a drop in an ocean the size of every ocean in the galaxy combined. How could a force this size possibly hope to face off against the Imperial navy? For that matter, why would the Imperials only send a few Star Destroyers to destroy the Rebel base if they knew it was their chance to crush the Rebel Alliance once and for all?
Now let's look at the navy. An Imperial-class Star Destroyer has a mass of 25 million tons (I got this from a disreputable source, but bear with me). An aircraft carrier might weigh around 70,000 tons. That comes to about 1/350th of a Star Destroyer. The world today has 22 aircraft carriers (again, thank you Wikipedia), which means about 1/20 of a Star Destroyer (I'm using rough numbers here). I've done some quick math and I assume we can double that easily with the rest of the world's navies; this brings us to 1/10 Star Destroyer. Going back to 24,000 worlds, this gives us an estimate of 2,400 Star Destroyers in the Empire. Wow.
Where are they all? We see none when the Death Star comes for the Rebel base on Yavin 4, and only perhaps a hundred at most at the Battle of Endor. Wouldn't the Emperor focus all of his Star Destroyers at the place he can destroy the entire Rebel Alliance--not to mention needing them for his own protection? Wouldn't he bring in at least half his fleet?
Then there's the question of the number of starfighters we see in space battles. According to Wookiepedia, the average complement of a Star Destroyer is 100 ships, of which 48 are fighters. Compare this with a modern aircraft carrier, which can carry more than 90 fighter aircraft. Consider again that an aircraft carrier is 1/350th the size of a Star Destroyer. Of course, a Star Destroyer also carries soldiers, assault vehicles, etc, but if even a tenth of it was dedicated to fighters, it should hold 35 times the number of fighters as an aircraft carrier (not to mention a TIE fighter is only about 6 meters and a TIE Interceptor 10, while an F-16 is about 15 meters, more than double the length of a standard TIE fighter). That means each Star Destroyer should be carrying more than three thousand fighters. Considering Earth is currently listed as having 28,000 fighter aircraft (a third of which is about 9,000, going by our earlier-used ratios), we see that there would be more than enough to equip those Star Destroyers. Phew! Take that, Rebel scum! So if there were even one hundred Star Destroyers present at the Battle of Endor (not unreasonable, considering the vital importance of the battle), there would be three hundred thousand Imperial fighters to contend with. Those TIE fighters might be crappy, but that's a lot of green laser beams!
By the way, can a 16 meter X-Wing ever really hope to destroy a 1,600 meter Star Destroyer? I'm not C-3PO, but I'm going to say the odds should be 100 to 1. And that's not including the Star Destroyer's fighters.
It could be argued that the majority of the Imperial army and navy are occupied with controlling the Empire elsewhere, and only a fraction is free to fight in any given battle. Some evidence for this would be the presence of stormtroopers on Tatooine. I would argue that those stormtroopers are only there to search for the droids and the stolen plans; local pacification is probably done by locally recruited troops, if it's needed at all, as in Vichy France. Surely the majority of Imperial worlds don't need a standing garrison. They should be able to focus on the major threat to their dominance. Even if there is rebellion on other worlds, an overwhelming defeat of the Rebels at Endor would destroy hope and go a long way to re-establishing order.
Not to mention there are no Star Destroyers or even more than a couple dozen fighters on the whole first Death Star? What in the world was the Empire thinking?
Let's be honest with ourselves: Star Wars is a story, not a simulation of real galactic warfare. The numbers just don't add up. As a story, it draws its influence from our modern experience: each planet becomes a city in scope of battle, so that the galaxy becomes the planet. Explained this way, we see why the battles are really on a planetary, not galactic, scale. The writers can only create a world as far away as their own imaginations can go... and those didn't go far from this planet we call home.
