Countries like China have a large number of soldiers because of the relative ratio of capital to labour in the country. China is a nation where the the ratio of capital to labour is small and therefore it is more cost effective to have more people to equipment than the ratio say that Britian has in people to equipment.
This ratio is not a static one. China's population increase is very low compared to capital increase and therefore as time goes on, its military will become more capital intensive. That is, China's military will have more equipment for a given number of soldiers.