The GAs goal is an on time departure. It's much easier to tell everyone no than to take the word of a passenger who says their bag fits. If they tell you yes than what about the next 3 passengers that swear their bag will fit too so its ok. I remember not long ago there was a poster asking AITA for carrying on a massive hiking backpack and taking up the entire bin but justifying it because, "it's the same size as a rollaboard with its handles extended!" Some people will try to get away with whatever makes life more convenient for them and not care about their impact on others. I know that is not your case here but the GA wont see it that way.