If you want to reduce the size of the files on disk for your database, you
can run repair, since while mongodb reuses the space from files you've
removed, the space isn't automatically reclaimed by the OS.  More info on
doing this can be found here<http://www.mongodb.org/display/DOCS/Excessive+Disk+Space#ExcessiveDis...>
.

You can run compact
<http://docs.mongodb.org/manual/reference/command/compact/>on a *single collection
*(also explained in link above)* *if you're looking to defragment extra
space that could be caused by a series of deletes.

As for whether or not storageSize is affecting the performance of mongodb--
it's only a value explaining the amount of free space allocated to a
collection.

And yes, understanding the concept of the working set (see here<http://docs.mongodb.org/manual/faq/storage/#what-is-the-working-set>) is
always valuable :-)