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
iPhone 12 and 13 minis are the same hardware with smaller screen and battery than their iPhone 12 and 13 (non-pro) counterparts. Are you referring to the SE or something? Even the SE has been getting upgraded processors with previous generation screens, cameras, etc.
The minis have been flagship specs in smaller packages. If the smaller battery thing was to be a knock on Apple for making it inferior, how do you suppose you fit a full size iPhone 13 battery into that mini frame with all the other internal hardware the same?