Were they to receive 4.7 million new volunteers, the North Korean armed forces would be less able to fulfill its overriding war objective: the forcible reunification of North and South Korea.
This is relevant, in that North Korean state propaganda announced on Thursday, that 4.7 million of its citizens have now volunteered to join the military. North Korea intends for this volunteer pledge to intimidate the U.S. and South Korea, and make the threat of war appear ever closer.
Yet in functional utility, 4.7 million additional soldiers would hinder rather than help North Korea's power projection capability.
First off, 4.7 million new soldiers represent a lot of mouths to feed, and a lot of uniforms, beds, rifles, and ammunition to supply. Where would the increasingly-strangled North Korean economy scrape up the funds and material to supply such a vast mobilization effort? Moreover, in the event of war, how would it maintain a supply line to so many personnel? This concern is accentuated in that many of the new conscripts would likely be drawn from wartime supply production roles.
North Korea might think that throwing an additional 4.7 million soldiers onto the battlefield would allow it to concentrate force against the smaller U.S. and South Korean battlegroups, but doing so would stretch its supply lines far beyond the breaking point. As Napoleon learned in his 1812 invasion of Russia, however well-trained or well-led (neither of which applies to North Korean forces) an offensive army is, absent effective supply, its warfighting ability evaporates.
Another challenge 4.7 million soldiers would pose is that North Korea lacks sufficient tanks, armored vehicles, and associated fuel supplies to support such a vast advance. Kim Jong Un's only chance of success in South Korea would be his rapid seizure of Seoul, and agreeing to a cease-fire in return for his ceased advance. But in order to take Seoul and credibly threaten a southern advance, North Korea would have to create multiple vast breaches in the U.S.-South Korean allied lines. That would require dividing already-stretched North Korean armored forces even further, and thus sacrificing a potential rapid advance with fewer soldiers, for a much slower offensive by many more soldiers.
Geography poses Kim Jong Un another problem here. After all, due to South Korea's mountainous eastern terrain, there are not many well-suited offensive approaches along which North Korea can funnel its forces in the first place. Correspondingly, as a lethargic North Korean invasion force struggled to break through congested choke points, it would come under relentless attacks from allied air and artillery forces. North Korea would risk a modern re-run of the 1415 Battle of Agincourt, where a far larger French army was annihilated as its compressed forces proved unable to advance under withering English arrow fire.
Facing overwhelming attacks from all sides, lethargic progress, and certain death, the commitment of North Korean conscripts would likely waiver.
This isn't to say that we should underestimate the risks of war with North Korea -- we absolutely must not. Nevertheless, vast numbers do not necessarily correlate with effective war-fighting potential.