I'm not sure about that. This could be due to the marketing/positioning i.e. Apple first switched to producing bigger phones (with more features/power) which caused people to buy these bigger phones (there were no smaller & newer phones).
After some complaints, Apple produced a smaller version of their phone but they market it as one with less features (smaller battery, smaller processor, etc).
If Apple devoted resources to producing smaller sized phones with comparable features as the Pro, the statistics might be different. To be clear, a larger phone will always have some edge but the minis should not be clearly designed as a step down version phone