My understand of the 1960's Spanish law which applies here is that the volume of luggage that is accepted as carry-on is fixed by the aircraft manufacturer (e.g. size of the overhead bins), and not the airline: the airline cannot arbitrarily decide the max volume of carry-on that passengers are allowed to bring (and worse, cannot selectively enforce a different limit for different passengers).

I can clearly see the slippery slope if rules like this were not be enforced.