You coould turn BBS into a CSO though i have no guarantees on how much that will save and it will slow loading times.
If you could remove FMVs etc. from the Digimon World ISO you should make a smaller eboot but I have no idea how to do such a thing.
i dont have bbs, but as far as installable stuff, it's usually in a seperate folder and 95% of the time it's a copy of files in other folders and can be deleted.
5% of the time though, it isnt and the game reads it from the umd if it's not installed.
so try deleting the contents of the install folder, *but keep a backup of the full version incase it wont run.