My 2 TB disk was close to full, mostly with FCPX projects and events. After converting to 10.1, I have a TB free. I had a lot of render files and other miscellaneous stuff from using FCPX in a less than precise way and apparently going to FCPX10.1 meant that those render files and other unneeded files were not carried over. In my case that is a good thing.
Also, a few original files turned up missing and I found them on a backup disk and 10.1 relinked them and it put the originals back where they should be.