I replaced the original headline from "The monster atomic bomb that was too big to use" to "The Tsar Bomba was Nerfed", hope that's okay. It points out what was, to me, a new and most surprising fact. Key quote FTA:

"Before it was ready to be tested, the uranium layers that would have helped the bomb achieve its enormous yield were replaced with layers of lead, which lessened the intensity of the nuclear reaction.

The Soviets had built a weapon so powerful that they were unwilling to even test it at its full capacity. "

Well, personally, I had to look up what 'nerfed' means. It's a video gaming term apparently? But you do have a talent for headlines. :-)

I wonder if they worried about igniting the atmosphere, like the Los Alamos people considered they might with a much smaller bomb.