In the treesize report, the Used space is shown as 36.1 gb but adding up the individual folders size gives only about 20gb.
My guess is its the hibernation file which is taking up space that treesize cant read.
Do you use hibernation? Try turning it off. Click Start button, type "cmd" without quotes, then click on the result. At the command prompt, type:
powercfg -h off
Hit Enter. This will disable hibernation and delete the hiberfil.sys.
Then run Treesize again or check in windows explorer.