Don't worry about it! I've never flown Southwest, but I have flown US Airways, British Airways, and Aer Lingus. When I flew, I known I was a size 24, but I'm unsure about my exact weight. Probably 270ish. I fit in the seat fine, and I was even able to put down the arm rest (although I only used it on one flight... all the other flights I had no one sitting next to me and I left it up). I also wasn't even close to needing a seatbelt extender.
No one ever commented on my weight, either. So I'd say you have nothing to worry about. It's all in your head
No one else will be thinking about your weight while on a flight to Vegas.
Have fun!