I would dearly like to see it happen, but it will never be (not ex U.K. anyway). The human rights brigade would have a field day with that one. Someone being made to stand on scales at check in and having to disclose their weight!!
At least one U.S. airline will make you pay for an extra seat if you are too rotund.
It can be quite an advantage being weight conscious. On a helicopter trip to the Grand Canyon each of the 6 pax were weighed to balance out the copter. As my wife and I were the lightest we got to sit in the 2 seats in front next to the pilot