"Hardly a Truman-style atom bomb, eh, and big and heavy"
Why would it need to be?


If you were within 150m of the blast of the tactical weapon mentioned above, you would be very dead regardless.
10,000 REM would likely even zap some of the cockroaches as well.
Have you never noticed Miniaturization, the trend to manufacture ever smaller mechanical, optical and electronic products and devices?
Rumours of suitcase bombs may be just that or may be true. We are not privy to such things.
Prudent planning incorporates the worst case scenario. A jolly good job you ain't in charge is it not?