[fredley]

This is pretty unenforceable, but could work if you counted passenger weight as passenger + carry on baggage, and merely set a limit on that (above which you have to pay a surcharge). That way you're not inherently charging heavier people more, but rather limiting how much baggage they can take as a result.

By setting this limit appropriately, you could ensure you're never forcing anyone to pay extra (as long as they're not morbidly obese), as long as they take an appropriate amount of carry-on baggage.