Mammals require an extensive respiratory surface because we have high energy requirements.Unlike fishes, reptiles and amphibians, we are warm blooded and maintain a constant body temperature, and this does take energy.Compared to reptiles we use more oxygen because we need it to convert food into body heat. We are also more active than most reptiles. Birds don't need as much respiratory surface as you would expect because their lungs have one way flow so every breath has the full amount of oxygen as fresh air. Our lungs are mostly filled with "used" air which is lower in oxygen content.
