It's a combination of structural and ergonomical.
Additionally, there is another one, which is present in the photo. Look at the door just behind the cart. If the cart was twice this height, it would not fit into the door.
It it were twice as wide, then it would be difficult to maneuver in tight spaces.
If you look at it, it takes about the same volume in space as an large adult person. The reasoning behind it, is that you want it fit more or less where the person fits. Additionally, if you make it too large, then you probably need to start thinking about motors to move it and steer it.