99% of people know what happened at Hiroshima and the awful destruction that followed, that percentage will drop like a rock when you start bringing in TNT or jargon like 'kilotons' then asking people to multiple such things by X amount.
If you read my first post, I do say measuring in kilotons of TNT also means little to the average person.
Mentioning a radius of destruction and then saying something like "it could completely destroy a city the size of Chicago", for example, would give people a clearer picture.
Anyway, after reading all the replies to the thread, I do suppose the expression is more about conveying a general image of a huge, devastating bomb leveling an entire city or area at once, along with the aftermath of it, rather than trying to give precise measurements for the level of destruction.
Beyond that it's just such a ridiculous amount of destructive energy that throwing precise numbers at people don't do it justice.