Nobody will interfere if they grab land and resources from Cambodia or Myanmar but size, distance and cooperation will stop them from doing significant damage to most of the world (including Japan).
The thing is, China has no interest in doing any of that. Even the US learned from the experiences of prior empires in not trying to "take over" countries. China learned even more from the failures of the US empire. You don't need to invade, you don't need to coerce, you don't need to manage them etc. Just buy them.