Tl;dr is that the amount of energy required to separate a mixture of gasses (U238 waste and U235 product) is roughly proportional to the logarithm of the ratio of the U238 percentage and the U235 percentage. So as your feed stream becomes more enriched in U235, it becomes much easier to do subsequent separations. This log relationship is an approximation, but arises out of the statistical mechanics of separating two mixed gasses and the resulting decrease in entropy.

Edit: a key point most people I'm guessing aren't aware of: centrifuges don't really care what you feed them, whether the feed is natural or 20% or 89% enriched, they just get increasingly more efficient so that a single "pass" through them produces a greater amount of separation as the feed stock becomes more enriched. They do a fixed amount of "separative work" each pass. The same machines can be used to enrich from natural to 20% as 20%-90% (with some relatively minor caveats), and in fact it takes far fewer machines to do the 20-90 step at the same rate as natural-20.