Even if you find a 15 MB file, it won’t open past the menu. PPSSPP itself needs ~60 MB RAM. GTA San Andreas requires ~256 MB of free RAM just to render the first mission.
The legend of the "15MB GTA San Andreas Repack" for PPSSPP is a classic piece of internet folklore, usually found on late-night forums and sketchy YouTube tutorials from 2012. Here is the story of that elusive download: The Phantom ISO gta san andreas 15 mb download ppsspp repack
While many sites claim to offer a GTA San Andreas PPSSPP Repack," it is important to know that GTA San Andreas was never officially released for the PSP Even if you find a 15 MB file, it won’t open past the menu