Here is my theory on Metru Nui's connection with Mata Nui.
There were two islands, one was known as Metru Nui. The other was uncharted and off the maps. On the one known as Metru Nui, there were six Matoran who liked to play games using Kanoka (or something like that) disks. Then some evil dude came and terrorized the island with evil creatures. The Matoran on Metru Nui tried to fight and then came the help of the Turaga Toa (with Vakama and Nuju). The Turaga Toa fought these creatures until they were too old to fight. The Great God, Mata Nui came down from the heavens and took them to the new island. The Matoran were dead, so he gave the Turaga Toa, now just Turaga new Matoran who liked to play Kohlii. Mata Nui promised that if evil came to this new island that he would send a new batch of Toa. The new Matoran (Takua, Jala, Huki, Maku) and the Turaga believed in this. The Makuta came to the new island, put Mata Nui to sleep and terrorized the new island, Mata Nui. Then, the new Toa (Tahu, Kopaka) came and saved the island four times from different Makuta threats.
That's what I think. It's a theory. Not a fact. Some of this theory was supported by the 2004 set list. Thanks!