"Little Boy" at 13kt is tiny by todays standards. What required a 10,000lb bomb back then could fit into something the size of a briefcase today
A relatively modern B/Mk-83 (designed in late 70's, largest gravity bomb still in current arsenal), weighs a mere 2,400lbs with a max yield of 1.2Mt (over 1000 times more powerful than hiroshima bomb)
A single Trident II D5 (sub launched SLBM) can carry 8-12 mirv warheads, each of which 30+ times more powerful than hiroshima bomb. A ballistic sub can carry 20-24 missiles, and we have 14+ ballistic subs currently carrying them; you do the math...
Single warhead unguided nukes are obsolete; precision guided multiple warheads in a single missile are far more practical.
It's easy to defend against a big lumbering bomber carrying one massive bomb; not so much against a dozen small warheads falling from space at 20+ times the speed of sound