/fill does not calculate number of blocks in hollow mode correctly

I expected when trying to do a 20 * 20 * 20 cube of glass to have it not calculate 20^3, but it actually used 20^3.

How to reproduce:
Do a hollow /fill on a cube that has more blocks than 4096 (eg. 10 * 10 * 10)
You would expect it to not show (8000 > 4096) but the number of blocks on a hollow 10 * 10 * 10 cube.