Well, OK then.From a psychological point of view, a box that is not full can cause people to feel they are not getting their money's worth. To this end one local shop will not stock box sets, for, the books out sell the boxes. A box set has to be special ordered.
From the point of view of one's wallet, a larger box weighs more and thus shipping costs are increased.
I rather have my shelves be neat and tidy, unlike my brothers's whose shelving looks like a jenga game.