sparse image keeps growing

[digSD]

Hope this isn't a dumb question:

Situation: I am doing a backup (clone) to a sparse disk image. (This is because the destination is another Mac on the network, so I believe I have no choice except to use a disk image.) [BTW, I am using 10.4.11 and have no plans to move to 10.5, in case that's relevant.]

Here's the issue: the size of the disk image grows significantly each time, even if very few files actually need to be updated (the image seems to grow more than needed). It gets worse if I actually change a significant fraction of source files: the image (destination) first grows to accommodate the changes, but then does not shrink when source files are deleted. So, very quickly, the image (destination) becomes too large for the HD on which it is located. This is true even if the destination HD is much bigger than the source HD.

I hope this makes sense... any suggestions, other than periodically deleting the image and then doing a full (rather than update) backup?

Armand