America has about 2 million. China I believe is near 4 or 5 million. However, males age 16-49 currently are at 314,459,083. IF china HAD to, they could probably whip up a 10 million strong military of very low quality.
With such a massive military, the amount of resources required to feed and transport these soldiers would be huge - forcing action to be quick, or otherwise destroy the Chinese economy.
For example, you see the trillions in debt we went in over our war adventures in the middle east. The pentagon managed to 'lose' and 'misplace' no less than 1 trillion a year.