Usually upgrading from one Windows version to another is not what I would recommend based on my personal experience. First, the system is never as fast and responsive afterwards compared to a clean install and merging the old registry to the new ones increases the chances to get weird bugs, etc... Also it kept a lot of files that it doesn't need from the previous Windows 7 so this is probably why you lost a lot of space.
As others already mentioned, my only advise would be to format the drive and reinstall Windows 8 with a clean installation. Also look at Cin19 advises to better optimize your OS for SSD and also save additional space. Another thing you could disable is the automatic defragmentation, I heard that SSDs don't need it and it would only reduce its lifetime.