The reason for high-yield weapons was to compensate for the inherent inaccuracy of first generation ICBMs and gravity bombs. The first ICBMs could easily miss their targets by a few miles, and so the largest ICBM deliverable weapon of the soviet union was nearly 20 Megatons, large enough to make sure they hit what they were aiming at. With advancements in precision, The US's largest ICBM weapon is 375 Kiloton, while the Russian SS-18 carries a unitary warhead of 800 KT. As accuracy increases, the need for monster bombs decrease.
The Tsar Bomba, a 100MT device that was scaled down to 50MT for the test, was so large it was impractical and never put into service. It was meant as a show of force.