ArtLady,
With the version of Iomega backup that I use, you have a choice between "1-step backup" and "The Works." The former basically prompts you to insert blank zip disks as needed and offers very few options, but it's simple. I don't know what, if any, compression is the default for "1-step backup." The latter has lots of options including level of compression: none, medium (optimized) or high. The higher the compression, the less disks you use, but the longer the backup process takes.
That being said, a compression ratio of -1:1 wouldn't seem to make much sense, regardless of the compression chosen. I wouldn't be overly concerned, though, as long as the backup actually completes without having to go out and buy a few hundred Zip disks.