I am having the same problem. Hard drive filling up! Without doing anything at all.
I am taking screen shots of memory size and it seems like every 10 minutes almost 80 Mb is filling up.
I upgraded from Mav to YS.
I a also having an issue of Home Folder name changing with numbers. My Home folder is called Bangla. With YS, it is changing to Bangla(2), Bangla(3) and now it is Bangla(8). I found a solution from the Internet deleting some preference files and not it might have been resolved.
BUT WHAT TO DO WITH THE MEMORY THING?!!