@Marz. Yep thats the util I like to use... Batch Zip Toolkit.
Once Ive zipped all the files individually I use WinRAR to zip em up into one big file...
btw.. I tried the 7-zip program and didnt get the bug you described. I noted there were two versions on thier site and I tried the BETA version. I didnt try the other one..
I compressed an ISO with 7-Zip and then re-compressed it seperately with WInRAR and didnt see any noticable difference so I will stick to winrar for now.. (the ISO was full of dms files which are compressed anyway so I expected this)
Anyway, Its looks like they are having a big re-organisation over at aceroms so maybe they are going to put all the Amiga A's in one / two files and then all the B's in one / two files.. That would be nice.