From the graphic we can see that the severe blast radius is approx 50% @ 1Kt to the 10Kt Radius. i.e. Yield to blast radius it is not linear.

Hiroshima was 16 Kt, which killed 90,000–166,000 people.
Nagasaki was 21 Kt, which killed 60,000–80,000 people.

I also have it from a good source that almost as many died from the fall out effects.

A low yield 1 Kt explosion would typically kill around 20,000-30,000 (blast and fall out) people in a high density population centre with many suffering severe burns.