Jpbarr - if you do have syncing turned on, those transactions won't disappear from your database immediately. An hour after the delete transaction has been synced to all of your devices, those transactions will be eligible for compaction.
You'll see the disk space get reclaimed once the next compaction actually happens.
If you don't have syncing enabled, then it's possible that you're seeing the bug that Curt reported. If that's the case, the applescript mentioned in
this forum post should help.