The power of a nation's labor force directly reflects wartime output concerning all manner of military industry - bullets, bombs, uniforms, spares, specialized field equipment, medicine, etc... Traditionally, and expectedly so, larger overall populations have fielded larger labor forces (as exemplified by the China, India and the United States). This listing does not take into account labor force quality or education.
Labor Force data through 2015.
Sources: CIA.gov, CIA World Factbook, wikipedia.com, public domain print and media sources and user contributions. Some values may be estimated when official sources are lacking.