They used to be bigger though. I think around 20 megatons.
This is important because "in theory" no one is allowed to explicitly target civilians, which don't require much accuracy, per the laws of war. Military targets are often in reinforced concrete or buried and so require a precise high pressure hit to crack. In practice, aiming points are chosen of military targets that are quite close to civilians.
You’ll do way more damage way more reliably with 20 1MT warheads than with a single 20MT bomb.
But it required reliable ballistic missiles and M(I)RV. If your delivery mechanism is bombers, then at equiv-tech you have to account for high attrition rates, and thus need large payloads to ensure even low success rates will do the necessary damage to the target.
https://en.wikipedia.org/wiki/Castle_Bravo
The larger than expected yield caused a lot of problems.