It's about the total number of PGs being created on one OSD at a time, and a ratio is a much easier predictor of that than the total number. Splitting a 16k PG pool to 32k PGs should be very easy on a cluster for which that's appropriate since it'll be spread out (and, importantly, the atomic operations are limited to creating 1 folder at a time); splitting an 8-PG pool to 8k is very bad since it requires an atomic creation of 1000 PGs on each of 8 OSDs.