There is basically no theoretical limit to the yield of a hydrogen bomb. It’s all about how much tritium you pump into the fusion reaction. There’s just not a hell of a lot of point going THAT big. There’s only so dead you can kill someone. They could have doubled, or tripled the Tsar Bomba yield if they’d wanted to - or increased it by a factor of 100. But once it gets that big - how do you deliver it? And presumably there’s SOMETHING about the continent you’re dropping it on that you want to stay intact. The continental plate, for instance. Or the atmosphere above it.
A lot of smaller ones can cause just as much ruin with less side-effects per megaton.
Thinking about this stuff for a living must be the most depressing job in the world.