Interesting post for me to read.
I had my current SSD in an old machine, and with W7, Cubase and HALion, plus a few other programs, I had around 67Gb of free space.
I then built a new PC, and used the same SSD, and after formatting it (was that a mistake - not sure!?!), re-installing W7, Cubase etc, I now have only 61Gb free space. In fact, I haven't even installed some VSTi's and other programs, so I reckon I would be down about 58Gb free space compared to my old machine. So basically, I seem to have lost 10Gb somewhere.
I use East Tec Eraser and System Mechanic to keep things tidy, and the East Tec Eraser can clean the free space and file-slack, so I have noticed a build-up of files reducing my free-space like you have seen.
I did read that when re-installing Windows you should never do a FULL format, only a quick-format. Not sure why. I would try a free downloadable program such as CCleaner http://www.piriform.com/ccleaner
to see if it can clean up your problem files that seem to be taking up so much space. I've used it a lot too, but just stick to East Tec Eraser at the moment.
i7-3770 on Asus P8Z77-V Pro, Kingston HyperX 16Gb DDR3-2133, with OCZ Agility 3 SSD. Windows 7 Pro 64bit. Cubase 6.5, HALion 4.5, Cakewalk Z3ta+ 2.1, Rapture & Pentagon v1.4, plus various Computer Music VSTi's. Behringer MX602 & Truth B2031A Monitors.