Yes it is only 5 times bigger than earth, but don't worry. the story will progress later on and I'll make sure to make proper world building and properly introduce new and bigger worlds.
[Ding! , The forest of Doom is located inside the Omega City. The world is 5 times bigger than the Earth which is the host's previous planet. However after a devastating all out war between humans all was left is the Omega City or it is called as the last city.]