Logistics. I don't think people realize that to move around army of even 10% of that 2.3 million, you require significant logistic support (sort of like upkeep in Warcraft 3). You then have to question which country has the access to most of the world's ports and bases outside of their own. Most importantly, you need to be on good terms and friends with these host countries. This is what China is going up against. A huge military resting at home means jack all if you can't mobilize it and upkeep it.