Another reason: thanks to advancements in Aerospace Technology (and not only, ig Electronics and so on...) not only can we now have more accurate warheads, but we can also fit more into a single ICBM (see MIRV). read more
You can't do this with a big weapon. To defend against a MIRV, you need eight times as much antimissile capability. This is very impractical. Many missiles could potentially carry 12 warheads. By treaty we only carry 8, but we could pack 4 dummy warheads to further reduce the effect of missile defenses. read more
Modern warheads are almost all between 50kg and 1000kg, with few being less than 50kg. Modern nuclear warheads can be either single stage or two stage. With some simplification; single stage weapons are your classic WW2 atomic fission bomb. read more
Number of nuclear weapon types in the current U.S. arsenal: W76 and W88 warheads for submarine-launched ballistic missiles (SLBMs); W78 and W87 warheads for intercontinental ballistic missiles (ICBMs); W80 warheads for the air-launched cruise missile (ALCM); and B61 (multiple variants) and B83 gravity bombs. read more