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?