Yeah but that wasn't a Big Planet Boss. It was more of a Space Station Boss, which would also cause a Big Planet to explode in the process. It was mainly a Space Station Boss, though.
I remember when I first played Fusion and saw the SA-X blowing the door and the elevator and revealing those cold eyes to the camera, I have to admit, I got really scared...
The SA-X was a brilliant idea! It's so great that there's finally a foe that Samus can't defeat right when she sees it. It's something she might actually be scared of. And what would be more scary than to face yourself in a weakened state, and the other in a 100% state!? Now we probably what the space pirates thought about Samus...