The clone Trooper thing is....complex. I'm working atm so i might add more later, but I will say: remember they weren't actually supposed to win a war. They just had to be seen to be fighting and slowly losing a war, to gain palpatine more power.

The only convoluted explanation is a unit is actually x many troops... But this is just a hand wave to coverup for GL's error, and still doesn't account for the numbers gap.

Look at it like this, in World War 1, the monthly casualty Rate during a major offensive was somewhere between 100,000 to 200,000 soldiers... For both sides... And this is a war fought on one area on one world where both sides had similar soldier availability constraints. Could you imagine the casualty rates using star wars weapons across even a handful of worlds, especially when your opponent can manufacture soldiers at a rate of thousands every few minutes (judging by the factory scene on Genosis).

Those clone troopers wouldn't have survived a month during world war 1...

End of rant...

Somewhere between 1/3 and 1/2 of casualties (which includes anyone unable to continue fighting, dead or injured) were due to disease. So to be fair, you should give the clones 2 months

A lot of answers have come from EU, and that is fine. It is all fiction so I am not saying they are wrong. I am not as familiar or as impressed by EU so I will try to keep things as close to the actual movies as possible.

As far as I can tell the entire rebel fleet was at the battle of Endor. You could make similar arguments about the battle of Yavin as well, it was after all THE rebel base. But I digress, the rebel fleet at Endor was pretty big. You could probably count the Mon Cal ships in the scene where they jump to hyperspace if you wanted, but I haven't. Some of those Rebel ships were pretty big. I had always thought that a MC80 (the largest rebel ship at the time) was slightly smaller than a Imperial class star destroyer. But their is a scene in the movie where a Mon Cal ship is clearly behind a star destroyer and still much larger than it. I have never seen anything to substantiate this in any Star Wars publication, but then again I am not super familiar with the EU. I will say that the Rebel Fleet by the time of the Battle of Endor was quite large, though the vast majority of it (if the movie is anything to go on) is corvettes and medium transport. I like to tell myself that they bolted some guns onto those transports before they took them on an assault, but purely support ships are important in most naval warfare, you just don't want them in battles.

The Imperial Fleet is a little harder to pin down. As far as I can tell the Imperial Fleet at Endor is Vader's task force. Admiral Veers is the same guy who made his way to command during Vader's elimination of everyone who outranked him in Empire.

I think it is reasonable to assume that this is by far the largest Imperial Task force, and most Star Destroyers travel alone. We see a few of them together (3 I think) in Star Wars, but even then they are their to help Vader hunt down rebels, not patrol Tatooine.

I also remember reading (I know I am dipping into EU, but it was unavoidable) That even with the Imperial's massive fleet there were simply not enough star destroyers to put one around every planet or in every system in the Empire. That being said, still huge. With the hyper drive capability of Star Destroyers and the fact that they can aren't just fighter or troop carriers, but also some of the most powerful gunships the galaxy has ever seen. Just being within a quick hyperspace jump means that the force projection capabilities of a Star Destroyer were immense.