The defrag is to have the file part located physically next to each other to speed up reads, and to keep free space in as few as possible places - speed up writes. But this performance increase is noticeable only for the standard HDD where the read/write speed depends on the disk RPM and the distance between the sectors that have to be accessed. The SSD is more like RAM where there's is not mechanical part that affects the performance of the I/O operations. For SSDs we have something called TRIM, but this is more on the OS end and server different purpose (but I may be wrong).
Anyhow - back to the issue: remember to disable hibernate. Next thing would be going though the folder structure and checking what actually you have on the disk. I recommend installing Total Commander and enabling the "Show hidden/system files". then you can check how much space each folder take (hit space to select and wait for a sec). You can do that using standard windows explorer.
Actually it may be the steam games that take so much place. I have CS:S and the CS:GO and the steam folder is over 11GB.
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