You know how Shannon entropy works in CS, compression and stuff? Atoms work the same way: their mixing entropy is that same x*ln(x) sum which is an extremely steep function near its boundaries. That's your non-linearity. That statistical entropy corresponds to macroscopic thermodynamic properties, enthalpy and work. The starting uranium atom ratios, 0.7%/99.3%, are a very unbalanced mixture deep into that non-linearity side.

https://en.wikipedia.org/wiki/Entropy_of_mixing

(The other half of it is that, as you progressively enrich, you start to discard the "depleted" part of the mass flow, and work only with the, gradually smaller, "enriched" mass flow).