"Every airline has limitations on the size, and in some cases the weight, of hand luggage. It's not simply a question of whether or not it will fit into the overhead locker."

It's all part of the Safety Calculations that check the max weight of the plane and to ensure that sufficient fuel is on board. Past experience has shown that cutting corners on this kills people - even for 'cheap' airlines - well, actually most especially for airlines trying to cut costs ... :-)

Weight estimations hadn't changed for decades, then one day a plane fell from the sky - they revised those estimations when they found US passengers were on average 20 Kg heavier than the old estimates said - as well as the luggage allowance was also now way out.

Changes were made on those estimates, and things are enforced more strictly than they used to be - also most especially for smaller planes, where the chances of rapidly exceeding the safety limits can be higher...