A 12lb grey is pretty decent sized. I've killed a few that pushed 15lbs, but it's rare. I've killed reds in the 15-16lb range, with a very few that 17-18lbs.
My biggest bobcat to date was just over 39lbs, but a good friend of mine traps, and has caught a lot of bobcats over 40lbs. The biggest one I've ever personally seen he caught in a snare, and it weighed just over 53lbs.
I read an old newspaper article somewhere on the net one time that I'm thinking was printed around 1930 or so that said a 76lb bobcat was killed in PA if my memory serves me right.