Remember the days when anything went with regards to baggage? Then restrictions were brought in on the dimensions and weights of suitcases. In the last few years the allowable weights have been reduced and the excess baggage charges have gone through the roof. This is because it is the weight (and excess volume) that is driving cost to the airline.
How long before we start pricing tickets by pax weight?
"Excuse me sir, did you pack your suitcase yourself? Great! Could you just step on these scales here and we will get you checked in..... Uhmmm I see you are over the no-charge weight .... if you read the fine print on your ticket you will understand that I have to charge you an excess weight fee of $$$$$. Thank you for flying with ( ) Airlines. Have a nice day!"
Or maybe they will just have a size seat at check in: your cabin baggage has to fit in here - and you have to fit in here! Whoops - there is overflow; that'll cost you!