Well, so Hiroshima/Nagasaki were 15-20kT weapons. While there was a while were the US and Russia were routinely plopping 5MT+ weapons on strategic platforms, these days its more down to 100-400kT range, so we're already sitting at ~one order of magnitude for most common nuclear device that'll go off near you. Granted, there will be a lot of them, but still.

Ok, so outside of the immediate fireball, the majority of immediate damage is going to be caused by overpressure, which will obey square distance law. So you're 20x weapon will only cause ~4.5x the overpressure at some distance.

So now its civil engineering of modern buildings vs whatever they had at Nagasaki/Hiroshima. I honestly cannot answer this, but I suspect that they could probably sustain twice the overpressure at least. So you're actual lethality (from building collapse) at set distance is only about ~2x despite the ~10x increase in weapon power.

Now, it's all kinda moot cause there's not going to be 'just one nuke'.

For your enjoyment and horror, http://www.nuclearsecrecy.com/nukemap/ you can watch how dead you'll be.