The scientists learned that tritium effect could allow more yield from less core materials which would eventually would lead to the ability to have the MIRV warheads on the large land- and sea-based missiles. The size reduction of the actual warhead was also enhanced by the use of smaller electronic components.
Yeah, cluster nuking!
Yep, and that’s why American ICBM’s ended up with much smaller missiles than the Soviet equivalents—the Minuteman II and III are very small compared to their Soviet counterparts of the late 1960’s to early 1970’s. It wasn’t until the late 1970’s the Russians started to reduce the size of their missiles, first with the SS-20 IRBM and subsequently with their mobile ICBM’s from the late 1980’s.