Hmm.. maybe the lens is having a hard time moving to the outer regions of the disc? That's the only thing I can think of, especially to address why game size would impact what works.
I'm thinking it may be related to the motor or the track that moves the lens back and forth. o_O More data means more is burned further out, and possibly your PSP isn't able to access those regions. Might want to take a look and see if there's anything obstructing it.
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