If you can't fit in a single seat for whatever reason (fat, pregnant, broken legs, excess muscles grown on steroids, unwillingness to let your pet or musical instrument fly in the cargo hold) you should buy an extra seat or book a ticket up front where the seats are bigger. Sure, that's more expensive, but that's no different from Levis charging more for XXXXXXXXXXXXXXL jeans than for a pair of M-sized 501s.

If your luggage exceeds size and weight limits, you'll have to pay extra. Why should the airlines make a difference for oversized passengers? Especially when spilling over into the adjacent seat turns flying into an ordeal for their fellow passengers?