alt Hacker News> Similarly to how the hard part is to cause a self-sustaining fissile chain reaction, and once you do making the bomb bigger is not the hard part.
I don't like this analogy very much, because in practice making a nuclear reaction is much, much easier than making a nuclear bomb. You don't need any kind of enrichment or anything, just a big enough pile of natural uranium and graphite [1].
Making a bomb on the other hand, required an insane amount of engineering: from doing isotope separation to enrich U235 to an absurd level (and / or, extract plutonium from the wastes of a nuclear reactor) to designing a way to concentrate a beyond critical mass of fissile element.
The Manhattan project isn't famous without reason, it was an unprecedented concerted effort that wouldn't have happened remotely as quickly in peacetime.