1) Rewrite the whole Bootleg in INNO Setup (Or any other programmable environment).
2) Make use of FreeArc (INNO Setup Script already available, but can be easily implemented into virtually anything), to save more space.
Example of magochocobo_ff7pack3.rar: This file actually has the same folder packed twice, which is again a waste of space
Original Size: 415 MB
Packed with FreeArc: 186 MB
ProofP.S. Without the duplicate folder it would've been ~100 MB less
I also experimented with Upscaled Magic files (data.7z.001 - data.7z.006 ), since it also has a moderate filesize. The results aren't as good as the one mention above, because 7z is already quite strong compression.
Size of 6 files together: 545 MB
Packed with FreeArc: 539 MB
ProofI think FreeArc would be good for those big rar/zip files.