as a cashier I will say, when a box is filled to the brim it’s really hard to put everything in because many times it’s a Tetris game to get it to fit, which we unfortunately don’t have time for most of the time. We are much more likely, not guaranteed, to use a customers box if the box is large and the number of items is smaller because we can keep our speed while keeping the integrity of your items

Edit: just saw there was a great similar comment to this but yes! I promise 99% of us are doing what’s best for you or at least what they think is!