We just lost a chicken to fatty liver syndrome but based on how her keel felt, even the vet said she would not have considered her overweight. She consulted some avian specialist vets who said there is a big debate happening over whether the keel is an appropriate place to check for over/underweight at all - that since they don’t fly much it does not give an accurate measure. Their advice is to use the keel to check for muscle mass, but use stomach and thighs of the bird to get a more accurate sense of how healthy their weight is. I would recommend either learning to check these other locations or take any seemingly heavy chicken to a vet and have them check.