*** NOTICE BEFORE MODDING ***
ALWAYS, ALWAYS BACKUP THE FILES YOU WILL BE MODDING BEFORE YOU MOD THEM, AS THERE IS ALWAYS A CHANCE THAT YOU'LL MESS SOMETHING UP AND RUIN YOUR GAME. I AM NOT RESPONSIBLE FOR ANY DAMAGE TO YOUR GAME OR COMPUTER THAT THIS MOD CAUSES.
Go to your Star Wars: Empire at War Game folder (usually C:\Program Files\LucasArts\Star Wars Empire at War).
Double-click the "GameData" folder.
Double-click the "Data" folder.
Double-click the "XML" folder. (NOTE: if you do not have an XML folder, you can probably get one from Gamefront, along with the XML files. There should be some that work with the patched version of the game.)
Find the "factions" XML file and open it with Notepad or Word.
Carefully scroll down until you see some text like this:
Change the number to whatever you want your population cap to be. (you will have to do this separately for each of the game's factions.
Save and close the XML document.
There you go! Have fun! This also works for Forces of Corruption!