I have the same problem
It happend to me with Mavericks, right before I wanted to make a partition I noticed that my 'other' storage suddenly was huge. But programs like OmniDiskSweeper etc couldn't find were the storage was coming from. After trying everything I found online I made a Time Machine backup and a clean install. When I put the backup back the whacked up storage was gone.
But after using Yosemite on a separate partition for about 30 minutes the other storage also went crazy. I was bummed but like 'okay I have enough space for temporary download so no biggie.' But now I can't install PB2 because I don't have enough space :')
I already reported it to Apple when I first noticed it so that was a couple weeks ago. But now I'm really bummed because I can't continue properly testing, I have to stay in PB1 untill the official release. Or do a clean install which I hate and I'll probably mess it up. This is my thread:
https://forums.macrumors.com/threads/1768394/
Hopefully someone will find a solution