I never thought Kevin Smith was all that huge really. I've seen much more giant people on airplanes before (even Southwest flights), who didn't have problems getting kicked off for being too big.
If they're really going to enforce this rule, they should have either a means to weigh someone or as someone else mentioned, a stand-alone seat to sit in before getting on the plane to see if you'll fit into one seat or not. In my opinion, they shouldn't seat someone, who ends up actually fitting into the seat with the arms down and seatbelt latched, then proceed to kick them off. I'd be pretty upset too.
My problem is that no plane is comfortable for me, because I'm tall. My legs are usually cramped the whole time flying, and during the flight I'm trying to find an out of the way place to stick my feet and try to stretch my legs, since they fall asleep constantly from being cramped up. Unfortunately, even buying two seats won't help that problem.