cannabineer
Ursus marijanus
You cannot pack a gigaton of yield into such a missile.Russia has an automated nuclear missile defense system.
If it detects an incoming nuclear threat, it will automatically retaliate against the aggressor, by launching nukes at them.
So there is no catching them off guard, or sneaking one by when they're not looking, in this scenario.
Also, Russia has the Satan II nuclear missile (aka RS-28 Sarmat), in their arsenal.
Which can allegedly level a target as large as the entire state of Texas with just one missile.