At a certain point bigger yields are less desirable than more, smaller yields. Picture an enormous spherical fireball, then picture chopping it up and using the "useless" slices that were at high/negative altitude to instead cover more ground. Once it became clear that they were past the regime of yield scaling, the Americans moved on to developing smaller warheads that could do just that: pepper targets and deliver more devastation per pound than a corresponding single warhead of high yield.

Vanity metrics are always more important to the second place player, and that's what Tsar Bomba was.