4" reflectors vs 6" and 10"

Hey guys,

Found a great hanging reflector for my 1000w HPS and MH bulbs. I even have a nice ballast and all the fixins for proper plant growth.

As a matter of fact, here's the URL to the exact reflector I had in mind:

Sunlight Supply: Yield Master™ II Classic 4 inch Reflector [904370] - Reflectors, Hoods, Shields & Parts - Grow Lighting Systems, Fixtures, Movers, Ballasts, Sockets, Reflectors & Components - Horticulture Source

Now, the tricky thing is, while I know the reflector here can work with my switchable ballast (MH/HPS). The only thing I don't understand is what difference there would be in the 4" rather than, say the 8" or 10". Obviously it would use a larger size of flexible duct, but that aside, what advantage is there to the larger size?

I was just curious if there was a reason to go larger or not. All commentary is welcome, thanks in advance for the insight!!

I would think the larger duct size is for more powerful fans. Some peeps hook up several lights in a row and use strong (cfm) fans to draw cool air though all of them...:)

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)