Correct me if I'm wrong,- but the physical dimensions of the luggage didn't change, - just the maximum it could weigh.
Policing the physical size of the carry-on would be more pleasing to me than it being a few hundred grams overweight.
It's so annoying queueing at the boarding pass check and seeing punters with blatantly larger bags and also more than the allowed number of pieces. That is the real piss off.
Wouldn't save them much kero, but would more align with their locker space porkie pies.