Sparse images: only use smart backup?

[macfeller]

Just today have I considered sparse images and have found my way back here. I am inclined to not bother with them. I just don't get the benefit. If I have it right, a *full* sparse disk image, when unmounted is the same size as a standard disk image of the same size. That is, presently, one facet of my backups is a disk image the same size as my boot partition. There have been times that partition has been damn near full and so the backup was, too. Were I to use a sparse disk image for such, when unmounted wouldn't it be of similar size? If so, and with the fact that it doesn't shrink when things are deleted without delving into Terminal, what's the advantage?

I got into this because I was under the assumption that sparse disks were compressed. If they aren't (and in my tests they haven't), again, where's the good?