The width of the green region represents an estimate of the maximum number of ancestors for in each generation back. This region is referred to as the "ancestor cone", based on this note by Wait But Why's Tim Urban.

For hundreds of years back in time, the cone appears to be a line here because it is dwarfed by the world population. However, eventually the doubling growth takes over and the number of ancestors begins to be noticeable relative to the world population.

In this case, we assume a value of 25 years per generation, and so every 25 years back in time, the number of ancestors at each generation will at most double.

The number of ancestors at a given generation back is of course bounded by the world population, and likely eventually matches it. In reality, due to pedigree collapse (intermarriage), the actual number of ancestors in each generation may be less than than maximum, and so the actual manner in which the ancestor cone may grow is uncertain.