Manpower fit for military service in India:
males age 16-49: 313,321,639
females age 16-49: 295,951,438 (2008 est.)
But you can't mobilize all of them. Because then you have NOBODY left to build your new tanks, grow food, etc.
Beside, here are the figure for India:
males age 16-49: 231,161,111
females age 16-49: 236,633,962 (2008 est.)
Pakistan
males age 16-49: 32,453,913
females age 16-49: 31,369,057 (2008 est.)
Vietnam
males age 16-49: 18,849,274
females age 16-49: 20,575,884 (2008 est.)
South Korea
males age 16-49: 11,282,699
females age 16-49: 10,683,668 (2008 est.)
North Korea
males age 16-49: 5,141,240
females age 16-49: 5,139,447 (2008 est.)
Japan
males age 16-49: 22.963 million
females age 16-49: 22,134,127 (2008 est.)
So, with just 6 of the neighbours of China, we reach a total of
males: 320 millions
females: 326 millions
vs for China
males age 16-49: 313 millions
females age 16-49: 296 millions
China may be huge, the rest of the world is even bigger.