This isn't necessarily the case anymore. At least not universally.
Big bombs and mirvs weren't necessarily for more damage, but primarily to ensure smaller, high priority targets got destroyed. (Because accuracy of ballistic profiles and fusing abilities meant pretty bad accuracy in the downrange direction, which is an incredibly interesting and far-reaching topic itself).
Point being, more accurate weapons means (for the US, at least) about 2/3s fewer, smaller bombs are needed to take out the same target list. (The math works out to be approx 4th-root required power needed proportional to radius CEP reduction.)
And fewer, smaller, more accurate warhead targeting plus upgraded variable fusing means a target near a city doesn't automatically doom the city itself.
At least for whoever's getting nuked by the US.
There's obviously still debate on if the cities themselves would be targets, but the treaty numbers are designed to make the opportunity-cost of bombing a city relatively high, meaning not enough warheads to hit other high-value military targets.
Counterpoint: Who really knows what Russia and China are actually doing?