There is a solution, but it's not that simple, and if you feel you haven't much experience with command line in linux, don't try it home.

Basically, you have to build an ubifs image out of your rootfs tar archive generated by backupmenu. The ubifs image must be compressed with zlib (there is a flag to use it). Then you reflash the image and your free rootfs space doubles or more. The subsequent times you restore your rootfs with backupmenu, zlib compression will be retained, unless you reflash the stock kernel or blank in any other way the rootfs.

Refer to the first message of the backumenu thread, and to the messages #571 to #579.

But then. if you have somewhat more than 30 megs, you're fine.