I'm a pretty big guy at about 6' even and just shy of 250 lbs, last time I weighed myself (which I'll admit was a while ago, and I've lost a little weight studying abroad). For the metric folk, that's 113kg and 183cm. I've always been able to fit into a seat without any problem. Usually, I even have a backpack under the seat in front of me. And you know what? I fit in just fine, armrest down, no bother to anyone else. The worst situation I've ever been in was AirEuropa, which had really wide armrests. It's not so much that I couldn't sit, just that it was a little awkward to get in and it was a lot easier to raise the armrest just to get in. Sure, a couple more inches would be nice (that's what she- oh, never mind) but for a flight that lasts a few hours, if you can't suck it up you can buy another seat.

Edit: Oh, and Southwest's policy is probably best when actually followed. You get a chance to "prove yourself" as being able to fit. If you need 2 seats, you pay for a second seat. If the flight isn't full, you get a refund for that second seat.