I disagree. It's not the key; rather, the design of the horn has more impact. (I know, here we go again. Hey, if we're going to have a more civil environment in which to have these discussions, then let's take advantage of it, dang it). Yamahas aren't designed for outdoor use like the DEGs and Kanstuls are. However, as I understand it, Yamaha is working on a new design for a marching euphonium which Cavaliers will be using at some point, and this will hopefully lead into them producing horns more accomodating to outdoor use, both in projection and durability.
IMO, it's not "volume," but "projection" that's an issue. I know of huge college marching bands that are loud as all heck, but don't project nearly as well as a smaller corps hornline and therefore don't sound as loud. The design of the bell and throat has an impact here. Compare a soprano and a trumpet side by side; aside from the extra seven inches of (cylindrical) tubing, the main difference lies in the larger throat size on the soprano and the lesser flare of the bell. Both these features are found on both the Bb and G sops put out by DEG and Kanstul. They aid projection by focusing the sound a little more than a normal trumpet does.