However, for the monsters, all of this was not a problem at all.
The military power of the monsters is indeed limited, but this does not mean Zhou Tian has no other ways to increase his strength should he want to. Although the population of the Bearman Clan is limited, the population of Zhou Tian's vassal tribes does not count towards the Bearman Clan's total. The territory of vassal tribes is akin to Zhou Tian's territory; thus, when Zhou Tian expands, as long as his vassal tribes take action or he supports another vassal monster, the problems he encounters would not become issues at all.
Of course, the military power of his old home cannot be mobilized easily. There are indeed numerous vassal tribes living on that continent aside from Zhou Tian himself.