Recently did the helicopter sightseeing bit in the Grand Canyon. Not only were we all weighed (as a helicopter pilot I can appreciate this!), but there was a large sign saying "All Pax over 285lbs will be charged a supplement" which seemed not unreasonable as we were talking 4-5 seat helos.
It's never arisen in practice, but when we travel, Milady Teeters (who is about 9 inches shorter and 4-5 stones lighter than me!) often comments how annoyed she would be to be charged for - say - 5 Kg of excess baggage, when our allowance is the same.
One can see her point.