It's been said before, but it's worth reiterating: planets often have really large defence fleets. You can find out exactly how large by opening the relevant resource (probably the spob resource) in ResEdit and checking the Defence Fleets value at the bottom. The way it works is a little odd: if the value is 1084, this means that the planet has a fleet of 8 ships, which it will send out in two waves of four (unless it's four waves of two. I haven't got the EVO bible with me right now). If it's 2008, that's 100 ships (first three digits, less 100) in waves of 8 ships per.
In EVO, some planets have small-ish fleets of about 36, some (ie. Earth) have fleets of hundreds. Even with a small fleet (ie. human renegades), if some of the ships are carriers their fighter don't count for the defence fleet limit.
Yours,
~R~