Don't trust TreeSize too much though. It is a good tool but it struggles if you use compression which you do.
You have compressed your Windows directory and TreeSize says it has compressed it from 17.3GB to 6GB. This is complete garbage as you will get a 1.7 compression ratio if you are lucky. Neither the 17.3GB or the 6GB are correct. Definitely the 6GB isn't anyway and the nominal 17GB is meaningless.
I did in fact discuss this with their developers and they said it was too complicated to deal with compressed files so they don't bother.
Look here - my program files directory is apparently compressed 88% from 2.6GB to 313MB (or according to explorer 890MB). My Windows directory is compressed from 18.3GB to 2.9GB (er no it isn't).
It is not trivial to decide how to represent disk use but WinDirStat does a better job if you use compression (but it is much slower). According to it my Program Files directory is 1.4GB (which is about right if I look from another system).
You can use any of these tools to find big files but don't count on any of them to accurately describe your disk use because they don't (not because they don't want to - it isn't easy to do and it depends what you mean when you ask the quetion).