So today you have 1 gram. No bomb. Tomorrow you have 2 grams. Still no bomb.
...
365 days later, you have 365 grams after spending ungodly amounts of energy to separate isotopes. AND STILL NO BOMB! Not even a small one. These scientists are just some bullshit artists.
52kg later: BOOM!
Not a very good analogy, because by the time you get 26 kg, I still have 71 years before you get the bomb.
But you know beforehand how much you need. We can measure and make predictions with accuracy.