You know, I don't want to seem like I'm taking Hellion's side at all here, and Beta Ray Billy is one of my favorite guys. But I also read that series a lot and paid attention to the fine detail because it used to get distorted a TON on the old board by some posters. So hear me out on this.
I am... 95% that’s not a moon in that scan. It's another planet. The solar system that Bill and Stardust fought in was drawn in a way that made the planets look closer together than they actually would be. This was probably done so that the artist could draw cool things like Bill throwing the hammer, Stardust dodging, the planet boomeranging around the nearest planet for a gravitational slingshot and connecting with the back of SD’s head.
Also, the destruction of that planetoid looks nothing like the destruction of an Earth like planet that happens later in the series. This is where Stardust tackles Bill and Asteroth to the core of the planet, and then some combination of them blows it up. (This experience also does some reasonable damage to Bill, dazes him a little bit, but ultimately leaves him conscious and kicking.)
http://s673.photobucket.com/user/gal...roth1.jpg.html
http://www.comicvine.com/images/1300-3879882
So you COULD make an argument that the celestial body Bill smashed was probably not an earth sized planet. It seemed to be an independent body so I don’t think it qualifies as a moon. No real way of definitely gauging the size though. I don’t enough about celestial bodies to say if we can surmise it’s size based on its apparent composition or anything, but the authors definitely made it less of a BOOM than the example later in the story. I still think of Bill as a planet buster myself.
THAT SAID, Bill totally one shot the thing. Charged up hammer strike, striking Stardust’s lance, let’s out big wave of energy. No idea what Hellion was talking about there.