Kinda repeating most of what Illusion said but....
If your thinking of using those miracle energizer/duracell AA's that hold something like 2250mah, believe me most of them don't hold more than 1600mah in most applications.
The reason most airsoft batteries are bigger than AA's is simply because you want the biggest possible battery you can to fill up your battery compartment.
The physically larger cells are better to use because they are able to supply more amperage.
LiPo batteries are popular because they offer a significantly higher energy density than NiMH or NiCd, meaning more amperage and more voltage in the same size of battery.
blah blah blah
there's a few threads of batteries if you want to look them up, and if your thinking about looking at LiPo batteries, check out the newbie tank sticky thread that "m102404" posted