So yes - a 1kt nuke is useless against them.  The Iranians always knew a stand-off was likely and seem to have planned accordingly.

I also think there's a psychological threshold around 1kt. Under that you can argue - no matter how uconvincingly - that the bombing is tactical and not strategic. And some people will nod their heads and believe you, because for a semi-buried detonation, the radius of 1psi of overpressure - which means broken windows and not much more - will be localised to half a mile or so. (Fall-out remains a big problem. But they probably won't be thinking that far ahead.)

The higher the yield, the less believable that argument becomes. Once you get over 15kt you're into Hiroshima territory, way on the wrong side of the red line.

Anything over 100kt would be horrendous. I don't even want to think about the reaction to that.

My point was that any nuke-based plan has a fundamental flaw. If the US wants to say 'Look - tactical!' the bombing won't work. If it increases the yield, any pretense at 'tactical' soon disappears and it turns into a much more dangerous game.