Population numbers don't translate into getting recruits for the army and shouldn't be thought of in this way.
50 million with a 400,000+ man army equates to almost 0.5% of the population engaged in military activity. To say nothing of the logistics needed to support systems to support this military apparatus and the yearlong training needed to replace any number of these men. This is an immense strain on the state, and as such every man would count.