If you know how to make H-bombs, it's not hard to make bigger ones. The Tsar Bomba did not involve any real advancement of the art of making nuclear weapons, it just added more fuel to already existing concepts. As a result it was a very ungainly weapon — it couldn't fit inside an aircraft, and a lot of care had to be taken for the blast not to kill the people flying the bomber.

Neither the USA nor the USSR ended up pursuing weapons larger than the Tsar Bomba, or even the size of the Tsar Bomba, really. The state of the art, even in the 1960s, was to make very compact, very efficient thermonuclear weapons — weapons that might "only" have an output of, say, 200 kilotons (so 10X the Nagasaki bomb), but could be fit into a small volume that only weighted 300 lbs or so. Then you could put things like that onto cruise missiles, MIRVed ICBMS, SLBMs, etc., and have a lot more assurance that your weapon was going to make it to your target than you would with a big, heavy, slow weapon like a Tsar Bomba.

To put it in technical terms, the damage caused by a nuclear weapon only scales as a cubic root. So to double the damage of a bomb, you need to increase its explosive output by a factor of eight. By the weight of a bomb increases nearly linearly with the increase of energy output, once you figure out how to make them fairly efficient in the first place. So more explosive bombs are necessarily heavier ones.

The "sweet spot" are thermonuclear weapons that give you about 2 kilotons of TNT equivalent for every kilogram of bomb weight, and can be fit into a small volume. These make up the majority of the US and Russian arsenals today.

The US did investigate — on paper — weapons far larger than 100 Mt; weapons in the 1,000s of Mt range. But they never built them.