You might want to recheck that. The average nuclear warhead in the US arsenal is approximated to be 33,500 kilotons (slightly larger than the well known B41). For comparison, the nukes used in/on Japan were 15 and 21 kilotons. 33,500 kilotons is large enough to destroy/kill everything in a 55-60 mile diameter. It would take about 1000 of these to DIRECTLY kill everything in the United States. Factor in the indirect damage (nuclear poisoning, fallout, etc etc), and you could kill everyone in the United States with far far fewer. India (for example only), has 1/3rd the area of the United States. It would take probably 100 33.5 megaton nuclear bombs to kill everyone in an area equal to the size of India, and it would likely kill a couple hundred million of people not in that area.
That's completely false, most modern missile-based nukes are in the hundreds of kilotons, like 100-500 kt. 33.5 megatons is larger than the largest bomb we've ever had in service, the B53 at 9 megatons.