So what I'm wondering is, if I didn't enjoy Ratchet & Clank: Size Matters for the PSP, should I even bother with the PS2 ones? 
 
See, I didn't like Size Matters at all. It was an ugly game with boring combat, boring platforming and boring boss fights. Nothing stood out -- every single aspect of the game was either below-average, or just average. 
 
I'm not trying to get into an argument about whether or not the game's good, so please keep those opinions to yourself. I just want to know if the PS2 ones are different enough to be worth a shot, even if I didn't like Size Matters.

I don't believe they are made by the same developers.

R&C:SM sucks monkey balls, the rest of the Ratchet and Clank games (for the most part) do not.

I think the original and Ratchet and Clank: Going Commando are both awesome. I felt Up Your Arsenal was a bit of a downgrade, though it did get higher review scores but that's no doubt because of the added online play. I imagine they'll still hold up fairly well.

the PSP version and the console versions are, besides some of the characters, completely uncomparable.

R&C: Going Commando is the pinnacle of the series in my opinion (though I haven't played the most recent PS3 release yet). I would definitely recommend trying it out.