i have not used nuclear rockets before, but if i guess what happen the damage was distributed among the targets.
the nuclear rocket still have an attack rating on them so an army especially the navy since the rocket have 1500 value against ships, can survive by being big enough for the damage to spread. Unreal.
if were going to do what you want we will have to remove the stat-line of the rocket and give it an instant kill feature, the problem is we are one-hitting the enemy army, which both realistic and OP as f*ch.
all we can do is increase the attach value and that will be it. period.