Actually, the robot IS that huge. It is just as tall as Earth is wide, on which you can't even see Russia if you're standing on Alaska. To be honest, I don't see why Mata Nui's chest can't look like this. Nobody said the domes were all at the same height, and as Mata Nui controls the gravity in his own body, the tunnel that connects the domes would be sailable. The Matoran might notice the sudden change in direction, but they're probably used to that kind of thing by now. Also, the island of Mata Nui is approximately the size of the robot's head, and the island is exactly 7.5 times the size of Metru Nui. IMO, it's perfectly logical to assume that the islands in "Greg's" map are massively enlarged. Especially if you look at the size (and proportions of) Voya Nui.