Yup. The size issue is a surface area:volume:problem. Neglecting the lid, heat enters through the surface so that, all other things being equal, the bigger the suface area, the faster heat enters. Of course, we're assuming the same shape box (A cube like yours is better than any other straight-sided, right-angle shape). HOWEVER, as the length of a cube increases, its surface area goes up as the square but the volume goes up with the cube. For example, doubling the length increases the surface area four times and the volume 8 times. Compare the volume:area ratios of the two and you'll see this double-dimensioned cooler will keep its stuff cold twice as long--if it does indeed start with eight times as much ice, though!