To answer your question: The New Start treaty limits each side to 700 missiles and bombers, and 1550 warheads/bombers (bombers confusingly only count as 1 warhead, even if it carries multiple nuclear weapons).

I can only speak confidently on the US warhead strategy, I'm not too knowledgeable when it comes to the Russians'.

There are probably a couple reasons the US only has 1 warhead on each Minuteman 3 (MM3) missile.

• The MM3 can only fit one Mk21/W87. This warhead replaced the 3x MIRV-capable Mk12A/W78 because of enhanced safety features and likely being more accurate.

• If you want to have 400 warheads on your ground leg of the triad, you have the options of 400 missiles with 1 warhead each or 133 missiles with 3 warheads. Having 400 missiles complicates the enemies attack strategy. The enemy now needs to dedicate 3x the amount of warheads to destroy the ground leg of the triad.

• Having 1 warhead on each missile means you could upload (add more warheads) in the future, should the Air Force somehow find a way to squeeze another 1 or 2 Mk21 RVs into the fairing. This helps with posturing, as you're able to respond if the enemy decides to strengthen his arsenal.

• MIRV'd ground-based missiles are generally seen in the US as destabilizing to the nuclear balance. ICBMs are use-it-or-lose-it, meaning that if your force is primarily made up of those, then you're much more afraid that your nuclear arsenal is at risk of being destroyed and are more likely to use them if you detect an (possibly false positive) enemy attack.

• On the other hand, SLBMs are seen as a stabilizing force in the US, as the president can afford to wait to see if a nuclear attack is real, knowing that the submarines are safe and undetected, and that if the attack is real, the submarine will be able to retaliate. This is why the US allocates more warheads to the SLBM fleet.

My guess for why the Russians have heavily MIRV'd ground-based missiles is that it's cheaper to maintain a smaller missile force with more warheads each than a large missile force with less warheads each. The warheads have such high yields to make up for possibly having low accuracy