Shrinking an iso to as small as possible, what are the methods?

[Special]

I'm curious what all can be done to a freshly ripped clean iso image to get its file size down to its absolute smallest size while still working with PPSSPP.

I know about cso conversion, but is that all that can be done to shrink the size? I remember back in the day when 1GB memory sticks were still expensive as hell and space was an issue for most, people would then rip out their games update folders, other language files, dummy files and even videos, all in the name to shrink down file size to get all their games to fit onto their card, and they still worked fine as ever on their PSP, I believe the tool UMDGen was mostly used for doing it.

Even just shaving 30MB's here and there can add up to a lot of extra free space for people who have 10+ games like me.

I'm posting this from work so I haven't done any experimenting my self yet to see what works and what doesn't, but I was wondering maybe someone here has done some experimenting and has figured all this out already, and of coarse would like to enlighten me on the methods used.