> An ADF is a disk image of one disk and always is about 900 KB big (forgot the exact number, but they're all the same size).
Nope, that's not quite correct. Most of the ADFs are 880KB in size, containing 160 tracks of disk data, but there is also an ADF format storing possibly long tracks in MFM encoded raw format (Look at the factor5 homepage, a lot of their old games are available there in this special format) with close to 2 MB for one ADF. In the case of long tracks, there is no (non-floppy-hardware-modifying) way of storing them back to a disk. Also trying to compress these files is in vain, because a) the mfm encoding and b) most of the data is probably already compressed anyway.