I've hit this issue a couple of times recently so I thought I'd share what I've discovered.
I've been using MonoTouch on my Mac Mini running OSX Lion (10.7.4) and I've ran out of disk space on my primary harddrive twice.
After a little research it appears that MonoDevelop is writing massive corrupted log files. I haven't figured out how to stop the problem but the quick fix to free up your disk space is to:
Go to (your harddrive\Users\(your username)\Library\Logs
Delete the MonoDevelop-3.0 folder. You may also want to delete log files for previous versions of MonoDevelop - this can all be done safely.
This morning I had two files over 25gig wasting space.
If I find a solution to stop the log files being created I'll update this post. Strangely this is only happening on one of our three primary developement machines so it might be related to another issue.