MacBook – Mac disk free space automatically decreases

To find out what takes up space, you can use Disk Inventory X, http://www.derlien.com/, which is free, or DaisyDisk, http://www.daisydiskapp.com/, also in the Mac App Store.

Both sparse disk images and the newer sparse bundle disk images intentionality do not reap/reclaim space until they need to. This is especially so for a sparse bundle - which is designed to reduce the network transfers when the image is stored on a network share. It certainly speeds up normal read/write operations since the system can assume compact won't happen when the files are changing. When you check things in disk utility, it perhaps sees that space could be reclaimed and does that for you. You are correct, that in normal use, it ends up with you using more space than if you simply put things on the normal file system.

You could convert your storage to a normal sparse image to get more traditional freeing of space when you delete files stored within the disk image.

Certainly read the first two paragraphs of the man page for hdiutil.

You can of course kick off a manual compact operation with:

• hdiutil compact /path/to/sparsebundle.dmg