SharePoint 2013 document library size bigger than the sum of file sizes

I have a user that on his my site put lots of files into a document library. The threshold for the site is 20 GB. The document library shows in the settings that it has used circa 18 GB. The problem is that the total size of the files (there are around 2000 of them) is 2 GB. I know that SharePoint probably counts also the size of the versions, but most of the files of this guy don't have more than 4 versions, (few have up to 7, but the files are quite small). So he is suspicious because 4 x 2 GB is around 8 GB but not 18 for sure. How do I explain him where his space disappeared? Is it normal, is there an explanation to it? Or is it some problem with our SharePoint instance?