And what about my kids that only weigh 20-25kgs (44-55lbs) and yet pay 75% of ticket and full taxes (and 100% on domestic flights!). F3G is right in that we are being sold space not freight kgs.
But I think that this is all one big (pun intended) conspiracy. Plane has 150 seats and 98% of pax fit in very easily. Carrier increases capacity to 180 (by reducing width & pitch) and "tight" fits climb from 2% to 10%. Then carrier starts forcing supersize pax to buy 2 tickets. Who loses: all pax (reduced space/overflow into my space/buying 2 tickets); who wins: the carrier (150 up to 180 paying seats)!! It is discrimination against all pax!!
Back to reality. The problem is that no carrier is going to lay down specific details of when a pax has to pay for a second seat as they will be opening themselves to the PC brigade. Instead they leave it up to check in staff and CC to address the situation. If the carriers had clear published guidelines the CC/PIC would never be sucked into the debate.