Macbook Pro Flash Storage Space Decreasing

I recently (09/2016) bought a 13in Macbook Pro Retina with 250gb of flash storage. I use the computer primarily for video editing and have been storing all libraries and projects on external hardrives. I recently have been experiencing problems on my iphone 6 so have been messing about with restoring it factory settings to solve the problems. It worked. im now noticing tho that after each restore of the iphone i seem to be loosing storage space by about a 2-5gb each time the phone is restored. I repeated this once just to see if it would happen again. I'm now tasked with finding out what is using up those 2-5gb on my computer and where i can locate those files. So far i have been unsuccessful. any tips on how to remove these unnecessary files from my computer would be greatly appreciated.

Cheers!