Brian, yes mass is used to figure BC. However, since we aren't calculating for BC but using known BC weight isn't important right now. Weight like I said is a byproduct of bore diameter and scale of the bullet. To get a high BC bullet your going to have to make a long bullet and that requires more material so naturally the bullet is going to be heavier regarless of material used to make the bullet.
Here is a good article by Brian Litz over on
Long Range Hunting. Basically he states that to get a bullet to match the 6.5mm 142 grain Sierra MK you can scale down to a 90 grain .223 and up to a 220 grain .308 Caliber bullet to match the BC. So for intents and purposes weight doesn't matter except to get the desired BC a person wants to shoot.
This is where bullet shape becomes important because not all weights are created equal. A 180 grain round nose bullet isn't going to have the same BC as a 180 grain spitzer boat tail bullet, so it will be affectd by wind differently even though they weigh the same. So simply put weight doesn't matter only BC does when your talking about bucking wind.