You make no sense. Like TM said, this isn't the PS1 and imagine changing a UMD every time you enter a different city in San Andreas. I doubt it's even possible to do that and if it was, it would get annoying.
Most of the size in San Andreas was the radio (not talking about landmass) so if Rockstar cut down on the number of songs on a radio station, then the PSP can take it. The amount of radio files, I see, took more space than the landmass of San Andreas.