I meant that 21 megatons is big enough to be among what the USA and USSR would have called their biggest H-bombs. (megaton, to be clear, is not a measure of ma

I meant that 21 megatons is big enough to be among what the USA and USSR would have called their biggest H-bombs. (megaton, to be clear, is not a measure of mass when talking about bombs, it's a measure of energy released - a 1 megaton bomb is equivalent to the energy released by a million tons of TNT, if such a volume could be detonated)