I find this problem all the time in Europe.
Folks fly long haul in premium cabins on airlines like BA that have generous luggage rules, then transfer to a low coster to continue their trip within the UK, and then show up with the premium pax long haul luggage on the short haul.
Not an excuse. But I think that's what often happens. No planning ahead to the low-cost short haul leg.
I've done this a lot of times, and plan for the short-haul on eJ or Flybe when leaving home, so don't run into that big luggage hassle on the second leg.