Yield Master II 6" w/600W HPS is what I am running and it is a nice hood. The duct size is one diff and the other main diff is the size of the reflector.
L x W x H
YM II 4"= 20 1/4 x 21 x 7 1/2
YM II 6"= 18 1/2 x 22 1/2 x 7 1/4

This will play into how the light is cast (ie footprint). A larger reflector will cover a larger space with light.(to a point)