iPhone Backup Restore Ate 30 GB of HD Space - How to Clean
The first thing to try is to delete any extra backups that might have been made in iTunes. Open the Devices tab in iTunes Preferences, see if there are any extra backups (it should list the device name and date it was backed up. Any backup that has a timestamp in the Name (i.e. "Device Name - 2012-07-03 3:12PM") was created during a device restore.

Select the backups you want to delete, click Delete Backup…, then confirm, then click OK in the Devices Preferences window. If you don't click the OK button, the files don't actually get trashed from the Finder.

If after that, you still haven't recovered all your space, check the ~/Library/Application Support/MobileSync/Backup folder (~/ means /Users/your_user_name/) folder. That's where the iTunes backups are stored. They're named using hashes, so you have to guess based on the dates which is which, but if you see more folders than there are entries in the Devices Preferences in iTunes, they didn't get deleted properly. You can delete them directly from the Finder, but make sure you know what's what before removing it.

Finally, if you're still low on space, try a tool like GrandPerspective, which will scan whatever folders you specify (I suggest starting with your user folder), and give you a graphical representation of the sizes of all the subfolders and files within. Pretty easy to find big files or folders taking up space. A likely culprit after a restore is .ipsw files laying around that didn't get cleaned up - that's the system image for iOS devices.

It's also worth checking to see if during the restore, your iPhone sync settings changed - it may have synced apps or media that wasn't previously synced to your computer.