Well, of course there are tons of bad games under 500MB. You could destroy me there, but it wouldn't matter, as people don't play them.
FFIV Complete Collection is about 859MB as an ISO. Prinny and Patapon aren't exceptionally large games content wise, and the former isn't that involved. It mostly just tests your patience. War of the Lions is a port (is it considered a remake?) of what already wasn't a very big PSX game. I wouldn't expect its size to become too terribly large. Metal Gear Ac!d was a fairly early release, and game developers aren't known for utilizing all the space there early on. Wipeout Pulse is a racing game, and there is never too much content there and unless it is a console racer like Gran Turismo (where the PSP only has a fairly watered down version).
Can't comment on Killzone Liberation, but the game didn't look that interesting anyways. :3
Regardless, that is a very small number of games. PSP games worth playing that are within the range I specified (and actually, a few are even larger):
Ratchet and Clank: Size Matters
Dissidia 012
Daxter
Ys 7 (just there actually)
FFIV Complete Collection (:3)
The 3rd Birthday
Persona 3 Portable
God of War: Chains of Olympus
God of War: Ghost of Sparta
Kingdom Hearts: Birth By Sleep
And THAT list just goes on and on. There are definitely more games over the 750MB mark than under when it comes to the PSP games worth playing. You don't find too many games outside of older PSP titles that are at or under 500MB that are worthwhile, or aren't PSP Mini's.