I'm guessing the user Library folder isn't visible, and so not included in your count.
The biggest thing there is likely to be backups for iPhones or iPads.
You can check what backups are being kept in iTunes, and delete all but the most recent.
Your Library folders are hidden by default in Lion and ML. To get to your /Library or /Users/yourusername/Library (also known as the ~/Library) folders in Lion and ML:Thank you guys!
You're right there's no library folder showed inside my user folder, how do I make it to show?