I used to get away with nearly 30kg sometimes (with 20kg) allowance, when flying with EJ from Luton. Of course I used to flash my svelte 10st frame at the check-in girl at the time, who would give me a knowing wink as she scanned the luggage tags with no questions asked. And being active and trim I was able to carry all that weight, materials for my daughter's dolls house then later her 3rd bike IIRC, all the way to the airport bus on foot at 4.30am, nearly 2 miles from where I was staying.
But in the debate between cases and carcasses I too think "total combined weight" should be taken into account - eg get 5kg extra baggage if you're 12st or less - it would be an incentive for people to get slim and consume less - the world's rapidly running out of grain on which to fatten all the animals for greedy people to scoff.