Hi, Phil -- thanks for your post!
Sparse disk images have been around for quite some time, although they're rarely talked about. Apple themselves leveraged sparse images to implement FileVault, so it's been quite well tested.
I don't believe they shrink 'automatically', but since FileVault images will shrink, there's likely a command that could be used to compress them.
I believe, using a combination of saved settings and our upcoming AppleScript support (see your other question for more), you'll be able to quite easily develop a model that will unmount the sparse image and keep it 'out of mind'...
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--Dave Nanian
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