Does the bore size of the trumpet really matter?

Yes, but bore size is only one piece of the puzzle! The bore size of a trumpet is only one variable that can impact how the horn responds and sounds for a certain player. Don’t shy away from a horn just because it’s says "medium large", “medium bore”, or “large bore”.  It depends on the balance of the player, mouthpiece, and the instrument. So, the bore size is only part of the equation to ultimately balance the resistance to match the player and their goals.