Confirm that. Defrag needs around 20% free space on the HD to do the job properly.
Last week I removed all the junk that Windows had stashed away in TEMP and in various alphabet-soup folders - I think it was backups of the configuration every time I switched the machine off. That got me from 92% full to 55% full, and the defrag went like a train after that.
There was one .TMP file in there that was 868MB in size.
Now tinkering with Linux and liking what I see...