Comparing the passenger weight to the baggage weight is apples to oranges (or perhaps french fries to baked potatoes?). As I understand it, fuel is only part of the picture --- by decreasing the overall mass of customer baggage, the airline can transport more cargo which means more revenue, and the airline needs less infrastructure/personnel to manage that baggage. Also, when baggage is over a certain weight, it's supposed to be tagged accordingly so that handlers know to get a buddy and do a two-person lift on it, which essentially doubles the manpower required to get the heavy bag moved. I don't think that the mere existence of heavy bags causes the airline to double the number of handlers overall, because not every bag is heavy (although what I saw being checked at MCO yesterday... sheesh... aren't you people ever planning on going home?!), but when multiplied across the system, probably does add up to a decent chunk of money.

The overweight passenger does consume more fuel than the underweight passenger, but the costs stop there. They are not taking space that could otherwise be used as cargo or sold to another customer (and if they did, we wouldn't have this thread--- it's a problem when they are taking space that another customer has paid for), and in the vast majority of customers, they do not require any additional labor to get them into their seat. They get themselves into their seat like any other customer.

I think that using the comparison between extra baggage charges and extra human-baggage charges is dangerous territory; I don't put it past the airlines at all to one day start adding a "human bulk surcharge" (to try and stay PC), and I think right now it's one of those things that no one wants to try, for risk of major negative backlash from the customers. I don't want them to think it's actually OK to do this. Remember when everyone rallied for the 3-hour tarmac delay rules? The unexpected downside there is being seen now in more cancellations (and I don't know about you but I'd rather get home after a 4-hour tarmac delay than sit on the tarmac for 2:45 before being sent back to the airport to be told the next open seat is 2 days from now). Let the airlines think it's ok to charge for passenger and carry-on weight the way they do for checked bags and we'll end up with an even more chaotic and lengthy boarding process which requires a hideously intrusive weigh-in, and unlike the TSA, no way to opt out. On every single flight, not just the occasional one where a COS really ends up encroaching into your seat.

Don't get me wrong--- I think if someone needs two seats, they should book two seats, and the GA's and FA's need to enforce it--- just be really careful with what we wish the airlines would do to fix this problem, because in the example of charging passengers by weight, I think this "solution" is worse than the problem it aims to fix.