[narwan]

As far as I know most sams are not meant to actually hit the target they're fired at but to explode near it and have the shrapnel from the blast do most of the damage. So the bigger warheads on sams would have a bigger impact on the chance of scoring a damaging hit than than on the actual damage amount (as the blast is more or less omnidirectional a bigger warhead means a bigger volume covered increasing hit chance but the amount of blast/shrapnel actually hitting the plane would probably not increase by as much).