We can consider the space taken up by each can is cuboid with dimensions 6.5*6.5*12.5.
If packed in a single layer, the box dimension would be 3*6.5 by 4*6.5 by 12.5. This box has a surface area of 2(12.5*19.5+12.5*26+19.5*26)=2151.5sq cm. If packed in two levels the surface area of 2(25*19.5+25*13+19.5*13)=2132sq cm. So the double layer packing uses less material.