>Normally you would expect effective contagion control measures to have an exponentially larger effect at a societal level than it does at an individual level

Isn't this dependent on the assumption that the risks are reduced uniformly across society? Lockdowns/quarantines are the obvious counterexample. Someone going into complete lockdown reduces their risk to practically zero. However a small group of people doing that won't have much effect on the societal numbers. Therefore the societal reduction is much less than the individual reduction until that R value drops low enough.