I wouldn't want some fat ass flowing over into my seat and making me uncomfortable, so I don't see anything wrong with what Southwest did. You have to take the other passengers safety and comfortableness into consideration. This world isn't all about you, and if you're the cause of it, then yeah, you should be kicked off. If you have issues with it then either don't fly, or get off your lazy ass and lose some weight. You'd only being doing yourself the favor in the later.
"Damn I burnt my marshmallow again"