It's less about yield and more about reachieving parity. It's probably a dial-a-nuke, though.
If they think they can survive underground, that causes an imbalance in MAD.
From Ivan's perspective, their Topol-M restores the parity that our missile defense shield upset.
I'd put my money on our shield before I'd put it on their bunkers.
I read a little about these a while back. Dont even remember the source.
I believe they are designed to be low yield devices capable of penetrating into deeply dug in installations and destroying them.
Just the perfect solution to Iran, North Korea, and other traditional enemies who have invested millions in underground military infrastructure.
I know most people get a knee jerk reaction to nuclear development, but it seems this is the only known way to destroy these types of installations. (I could be wrong, however...havent been keeping up lately.)
What was the line from Dr. Strangelove? Mr President Ve VILL HAF a MINESHAFT disparity!
Seriously, why not just trot out the Neutron? That radiation will kill fifty or a hundred MILES into the ground and you can take over in a couple of days without even messing the floors in the bunkers.