Has to be environmental? CO2 vs O2 concentration; humidity and heat; calorie density in ground vegetation. All are different, and may have contributed to limits

Has to be environmental? CO2 vs O2 concentration; humidity and heat; calorie density in ground vegetation. All are different, and may have contributed to limits on maximum grazer size. Witness the brontosaurs. Long neck so they could remain relatively motionless and strip their surroundings of nutrients without expending many calories. Such an environment today where everything in a 20 or 30 foot radius is edible may be rarer.