The is famously small. A full, non-merged set of all working games is roughly 2.5 to 3.5 GB . Why? Because this version does not emulate hard drives, Laserdiscs, or complex 3D polygonal games from the mid-2000s. It focuses exclusively on the golden era: 1980s and early 1990s arcade boards (Z80, 68000, etc.).
Because the PSP was such a popular device for emulation, an entire ecosystem was built around the 0.37b5 ROM set. People created "clrmamepro" dat files to audit their sets specifically for this version, ensuring that the files matched the checksums expected by the emulator. Even today, if you download a "FinalBurn" core on a retro handheld, you might find that it defaults to supporting the 0.37b5 set for older games. Mame 0.37b5 Romset Download