When you don't have an active filesystem, you mean not a lot of data is changed in between taking snapshots. Data changes within the different snapshots are carefully monitored by WAFL. When you delete a snapshot, WAFL will go through it's file system and only those blocks not used anymore by the other snapshots will be deleted. So in your case, not a lot of space is freed.
As you are a newbee it's important to understand how the snapshotting is working within WAFL. This is different then other vendors are doing snapshotting!!! Please have a look at following links.