I've been working on a fictional world called Solum. On this planet are two continents. Is it grammatically correct to call the larger continent Soli Major and the smaller Soli Minor? I'm not confident I've declined Solum correctly.
You have a lot of creative freedom with that, and you could probably generate some interesting conversation in our chat with it. That said, here would be my answer:
Your declension is correct. Soli is genitive and major is nominative. Maior probably has an implied (pars). This is probably the most generic and understandable translation.
Maior could be declined "Majus" for an adjective agreeing with Solum. Depending on the setting of your imaginary world, this might sound cool: Solum Majus. Same with minor. It could be Solum Minus.