Defrag increases "used space" as reported by command-line defrag -

Why does defrag increase the disk's "used space"? This happens with both the
GUI version, and the command-line version. The command line flag "-w"
results in a greater decrease in used space. I used (command line) defrag c:
-a -v to generate a report, ran the GUI defrag, and then reran a report. It
seems that the GUI version repeatably increases "used space" by about .7 GB.
The "free space" value sometimes doesn't show a decrease, due to the lower
precision of the number (no decimal values), and the amount of fragmentation.
The command line version increased "used space" by 1.37, for each of two
runs that for which I saved the analysis report. The "free space" value IS
reduced for the more aggressive defrag:
"defrag c: -w -v. Is this additional used space usable, or is it lost
forever, or until the disk is reformatted?