I have a 2009 MacBook Pro into which I've put a 750GB SSD hybrid drive. Works great.
I recently deleted lots of files, and moved my available capacity from 80GB to 120GB.
Then, I deleted a second non-admin user account I'd had for a while and for which I had no further use.
After deletion, I am again down to about 80GB of space. I had selected the "don't save user files" option when I deleted the user account, by the way.
I'm very perplexed and can't think where this extra space has disappeared; I had expected to GAIN about 40GB as there were several big video files on the deleted user account.