Snapshot Size - Any Way To Compress

I've activated snapshots on my Filers and I've noticed that some snapshots are 20+ GBs. Is there any way to scale this down to a few MBs? I don't have enough disks to expand the volumes any further.

Also, what is the fractional reserve?

Snapshot Size - Any Way To Compress

There's no way to compress snapshots; when a snapshot is created the blocks in use become read only and if the data is deleted from the file system the blocks are still frozen but the space used by those blocks is allocated from snap reserve rather than the active filesystem. Therefore in your case a snapshot has been created and then at some point 20GB+ of data has been deleted from the active filesystem. If you're having space issues then you'll have to revise your snapshot retention and look into other options such as archiving to tape.

Fractional reserve is best explained in the documentation, these pages should cover it: