The atomic pile that was built under the bleachers at Stagg Field in Chicago was as big as a house. It was designed to be a sphere, but when it became clear that it would become critical long before they reached the top, they truncated the upper courses of carbon blocks and uranium slugs, so that in the end it was shaped more like a doorknob as big as a house.
It needed to be so big in order for the graphite moderator to slow the U-235 emitted neutrons from the tiny fraction of that isotope in the natural uranium used to fuel the pile enough to have a decent probability of being captured by other U-235 nuclei, and thereby create a chain reaction.
I’m given to understand that German graphite contains a wee bit, just a whisper, of boron.
Boron sops up neutrons, and those missing neutrons skewed the data just enough to convince the German scientists that a deliverable bomb was impossible.
When the thing was getting ready to explode, Heisenberg had no idea what to do as the thing went out of control. That was pretty much the end of any serious German work towards a nuclear weapon.