Looking at it from a security stand-point is a great way to go about answering this question. I agree that regulating seat width would be an interesting idea, but I think airlines are all about making money, and I have a feeling that bigger seat width equals less seats, which in turn equals less profit.

Regarding airplane seats - I was stuck on a plane once, next to a 300 pounder... His fat spilled over into my seat area, and I had to sit for 6 hours with this guy's belly and thigh blubber squishing me. Very unpleasant. When someone gets to that size, morbidly obese where they really do require two seats (but only buy one), they should be made to sit in special larger seats which cost more money because not only would it be fair for the people stuck sitting next to them - it also becomes a weight and balance issue for the airplane. People that size equal a person plus two baggage allowances - in addition to the baggage they're normally allowed. So while I have to pay $100 in "excess baggage" fees to bring my guitar along when, as a musician, I need to, this person carries the equivalent of several guitars, in blubber, free-of-charge. Not fair. When people check in, they should have to stand on the scale with their luggage.

I'm not anti-fat people - I could stand to lose a few lbs, but thankfully, I can still be contained in one seat. I would be ashamed if I couldn't, and mortified at the thought of squashing another person for an entire flight.

http://www.irishhealth.com/article.html?id=4339

Before having the solution of ticket charges and the size of seat arrangement, THE UPMOST ISSUE is the safety problems due to obese passengers:
Size of the life jacket, blocking the inner seat passengers etc.