NES Samus

[ProtoTroid X]

The point is that she's on a black background anyway so you won't see it
I'm pretty sure that's just to give contrast to make it easier to see against the white...?

You pretty much hit the nail on the head. Although that's not the only reason. It's also something of a...consideration of other possible uses of the sprite. Without the outline, the sprite would look pretty terrible on a lot of other backgrounds. In a normal NES Metroid setting, the outline isn't needed, but in some other settings, it might be necessary.

But I suppose that it does look terrible. I'll see about making it a little thinner, and shrinking the back...jet...pack. Whatever that thing is. Never seems to have much a use.