Air France measure passengers at check-in - in front of everyone - if they think they are too big for one seat and then chage them 75% of a second seat.
“They will be charged 75 per cent of the cost of the second seat, which is the full price excluding tax and surcharges, on top of the full price for the first. The decision has been made for safety reasons. We have to make sure that the backrest can move freely up and down and that all passengers are securely fastened with a seatbelt. People who cannot fit into a single seat will then be fastened by slotting the belt tip of one seat into the plug of the next, stretching over both seats. "
How humiliated would you feel? It's horrible.