As mentioned earlier in the topic, the problem with the GSR’s height is less that it’s realistic, but more that it breaks any sense of scale. Ignoring the consequences of two planet sized robots duking it out on Bara Magna, that’s a whole other headache, let’s focus on the fact that none of it makes any in-universe sense.
The Great Beings built Mata Nui in secret. Sure they were capable of great things, but they built a planet sized robot in a couple of months. Said robot must have taken off from spherus magna, and nobody noticed. There was also a prototype that walked around before collapsing. Nobody noticed that, either, or questioned why there were giant hunks of metal in the desert.
Oh, and the tribes of Bara Magna lived in said pieces. And when Mata Nui united the tribes, they pulled the pieces together. With rope. Pieces that, even if you say the GSR was to the scale of Europe, must have been the size of countries. It’s literally that scene from Spongebob where they push Bikini Bottom somewhere else, except done 100% seriously.
A similar problem happens with Unicron in the Transformers franchise. There’s a scene in the movie where Unicron picks up Galvatron and swallows him. Fans have pointed out that even if Unicron was the size of Earth’s moon, Galvatron – a regular cybertronian – must have been the size of Great Britain.