The shelves have the smallest surface (4x3) but also the smallest capacity: they can only hold 9 items.
They have the same cost to upgrade and gives the same energy as vertical racks and mannequins, they just have the disadvantage of this smaller capacity.
If we were to increase its width by one extra square (so 5x3), they would be able to fit 12 items (see picture).
Moreover, the lvl 6 shelves (which would then be 5x2) would also fit 8 items, the same as the vertical rack and mannequin.