[GraemeL]

Castle Bravo was a ground burst and produced a crater about 2Km across. The debris from the crater ended up as fallout.

Tsar Bomba was detonated at an altitude of 4Km, the shock wave prevented the fireball from reaching the ground and producing a crater, so it produced less fallout.

[Andrew_nenakhov]

That's clearly wrong. Debris from a crater is just what it is: harmless ash, with no dangerous isotopes. See [1] for the correct answer.

[1]: https://news.ycombinator.com/item?id=28479051

[user-the-name]

Neutron flux from the explosion will turn the atoms of surrounding harmless materials into heavier isotopes that are, in fact, dangerous.

[pfdietz]

And more importantly, the fission products from the bomb condense/agglomerate onto debris from the ground, allowing them to fall out more quickly.