> Both are full, but one will require fewer miles driven

That isn't really true, though, because these houses exist in the real world and aren't just buckets to be visited.

Your conclusion would only be true if houses are all equidistant from the warehouse. That truck with the 100 packages for 70 houses is going to have to drive by more than just 70 houses to complete its route. It would not add additional fuel or miles to stop at those houses while it goes.