I kind of remember reading that somebody involved in the creation / testing of that bomb made a rough estimate of the area covered by the fallout and decided to ditch the U-238 damper on the second (third?) stage.
Also, getting the plane dropping the bomb to a safe distance before the explosion was kind of an act as it was, if the bomb had gone off at twice the yield, it probably would have killed the crew.
(All this is from rather fuzzy memory, though! If somebody has less fuzzy information, I'd be happy to stand corrected!)
From my own fuzzy memories that's correct - a full 100MT device would have been a pretty awful weapon, I've seen it described as something that could have utterly destroyed Belgium but that would have killed people in Russia with fallout.
I seem to recall that the Tsar Bomb had multiple tertiary stages?
Nitpick: it's tamper not damper.
Also without a fissioning tamper round it's secondary and tertiary stages the Tsar Bomb actually was a "fusion bomb" in that a very high proportion of its yield came from fusion. In most H-bomb designs most of the yield comes from the fissioning of the tamper round the secondary.
With bombs of this type you can chain things on several times and really amp up the yield. The theoretical limit, if there is one, is absolutely nuts, like 500MT or more.
The problem is that beyond a certain yield most of the explosive force punches up through the atmosphere and is wasted. Apart from shock and awe, not sure what the utility of something that big would ever be.
Additionally since a spherical explosion of double the diameter takes eight times as much energy to create, you quickly run into diminishing returns. Lots of smaller bombs is more effective, hence MIRVs: https://en.wikipedia.org/wiki/Multiple_independently_targeta...
Nuclear weapons are so awesome and so utterly insane at the same time.
Also, getting the plane dropping the bomb to a safe distance before the explosion was kind of an act as it was, if the bomb had gone off at twice the yield, it probably would have killed the crew.
(All this is from rather fuzzy memory, though! If somebody has less fuzzy information, I'd be happy to stand corrected!)