the US kept developing larger and larger nuclear weapons throughout the 50s
What's funny, is that at some point someone sat down and did a little math, and realized that there is a vast scale of diminishing returns when scaling up anything that explodes. So the modern nuclear arsenal from every nation that has these weapons is made up of significantly smaller yield weapons than were tested and deployed in the 1950s.
The bulk of the US arsenal are "dial-a-yield" devices that top out around 450kt, because they are easier to lift and guide where you want them. Or, and you can fit multiple of them on the same rocket you used to have one big ass 5Mt warhead on. Missile crews at Vandenburg AFB aren't simply trying to put one of these things inside a neighborhood - they aren't happy unless their test "warhead" can actually hit an oil drum with a target painted on it from 9,000 miles away. It's not good enough to just put a hot one into a city - they want to put it exactly above some munitions storage facility, or an air field, or a naval base. And they drill on it.