the only reason why ps2 have slightly better graphic is because they have much bigger storage medium, and also the downclocked chip in PSP. We have not yet seen any PSP game coded at 333 MHz yet do we?
PSP graphic core is having more hardware supported instructions than PS2, as many PS2 graphics instructions are on software emulation, and also PSP has smaller resolution to output to, thus they can use the remaining resources on improving the quality of each pixel instead.

kenot emulate ps2 lah, where got enough space unless using 8 GB mspd
ps2 and psp CPU is a bit different, ps2 relies most of it's 2 vector units, while psp got dual cpu configuration (one is main cpu, another is media engine, it also got vector unit on it's own)

PS2 produce more GFLOPS than PSP, but then PSP has included more hardware instructions specific to 3D graphics.

I can say CC:FF7 characters are rendered at much smoother, less polygon looks than FFX, not sure bout FF12. And its not even coded at 333 MHz.