Two thoughts:
1. A few years back at security (in Chicago at least) on the x-ray machine entry belt was a frame that the baggage had to pass through to get into the machine. This helped minimize the size of baggage that could be carried on board;
2. Last November in South Africa I could not get to the check-in desk without first having my bags weighed. If the bag was over 7kg it was labeled as such and had to be checked in. And at the gate was a 2nd checker with a mobile scale just to be sure you were not cheating - and they weighed all the carry on again. If the bag was over the 7kg, they took the bag from you!