From what I understand, increasing the power of large hydrogen bombs had long reached the point of diminishing returns. Basically, you were just blasting a larger hunk of atmosphere into space.One of the reasons the Soviets didn't replicate the Tsar Bomba was because it was too risky for the bomber. I don't know how powerful was the Mk17, but it is, surprisingly, possible for a bomb to be too powerful. The Soviets decided that line was crossed with their Tsar Bomba. I think the USAF (perhaps surprisingly) figured it out before building one.