What they should really do is charge customer based on total weight. As someone who is a fit and healthy 75kgs it drives me insane that I could get charged for an additional 5kg in baggage yet someone who weighs 120kgs doesn't have a problem.
If you're 75 kg and travelling with 28 kg of checked luggage (5 kg over the minimum allowance), I would say you are travelling with more weight than the majority of others. I weigh more than you, and indeed more than the average person (I'm just under 90 kg), but I rarely travel with any checked luggage. So are you saying that you should have to pay more than me, given that you are travelling with more weight?