Q: Disk Drive Full

Don't really need to defrag a mac most of the time... http://www.macworld.co.uk/how-to/mac/how-defrag-speed-up-mac-os-x-2016-3600241/

I believe MobileSync is the location for iOS device backups created within iTunes. You can verify if this is correct by opening iTunes and choosing "Preferences." I can't recall the exact name of the tab in the iTunes Preferences window, but it should be something related to "Devices" or "Back-Ups." Each iOS backup managed by iTunes should be listed separately by device and the date/time the backup was created.

Yes, MobileSync is the folder the holds these backups. You can go into iTunes, as described and delete the older versions.

If your drive space issue persists after following the recommended steps for re-indexing Spotlight, you might want to try booting you Mac into Safe Mode. During a successful start up into Safe Mode, OS X will run (in the background) certain hard drive and file system verification and repair processes (as needed). While in Safe Mode, you could re-check the disk usage as reported by OS X.

OS X El Capitan: Start up in safe mode

Another option would be booting into Recovery Mode. One of the main options while in Recovery Mode is Disk Utility, which can be used to repair potential issues with your Mac's startup drive.

About OS X Recovery - Apple Support