Same thing for me but this time it was Alien Breed. Couldn't be bothered setting up a transfer cable just for a couple of disks so gave up in the end.
I asked exactly the same question on this board you've just asked and was told there was one program that supported multipart archives BUT the person who told me couldn't remember what it was called!
Suffice to say I didn't find it
You have set the zip compression to the best / highest setting? This may help if you haven't set it as most programs default to normal (which is a trade off for slower computers of speed to compress vs file size) - What program are you using to compress?
Can the SWOS disk images be read through workbench? If so you could take the files from the disk image, stick half on the PC floppy, copy the contents of the PC floppy to your Amiga floppy disk then transfer the remaining files in the same way.
(That doesn't make much sense but I think you'll get the gist of it!)