James 1077, your insurer uses mortality tables to price the rate you pay. If you were not overweight the chances are they wouldn't have requested you to attend for a medical. The insurer uses the medical to identify other possible risk factors which combined with your BMI could have made you uninsurable.
Standard passenger weights for a max seating capacity of aircraft (inc crew) of 300-499 adult male is 81.4kg, adult female 66.3kg, infant 0-3 years a, child 4 to 12 at 41kg an adolescent male 13-16 yrs at 60.50 and female at 54.7kg. For those of us who like the idea of more reasonable airfares having some sort of airline fat tax isn't an unreasonable suggestion.
Look at Wizzair. They wanted to reduce airfares so they are going to try to introduce a fee for cabin baggage. Don't Spirit and Allegiant Air charge over there?
Isn't weight an important part of airline economics?