No, I do not agree with Kimaya. The volume of the rectangular prism is 2 inches x 4 inches x 6 inches = 48 cubic inches. To fill the rectangular prism with 2/5-inch cubes, you would need 48 cubic inches / (2/5) cubic inches per cube = 120 cubes. To fill the rectangular prism with 1/5-inch cubes, you would need 48 cubic inches / (1/5) cubic inches per cube = 240 cubes.
So, it takes less 2/5-inch cubes to fill the prism than 1/5-inch cubes.
Another way to think of it is that the larger the cubes, the less number of cubes you need to fill the same volume, as the cubes are larger in size.