From what I understand, a regular PSF file contains both the instrument sound data and the sequencing in each file, whereas a miniPSF has the instruments in a separate file (I think it's psflib) and each miniPSF file just has the sequencing data. The PSF method would allow for more detailed and varied instrument sounds but the overall size of all of the files is bigger. If you tried to do something like that with miniPSF, it probably wouldn't fit into the sound RAM unless it was split up, and then it might have to hit the CD-ROM a lot.
FF7 has a miniPSF soundtrack and is 821K on my computer. FF8 has a regular PSF soundtrack and takes up 25 MB on my computer. FF8's songs sound a lot closer to what you would find on a CD than FF7's.
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