Why do mammals produce more carbon dioxide than insects?
As you said, endothermic animals such as mammmals or birds consume more energy and therefore expelmore CO2 not only because of a higher betabolic rate or to keep warm, but also because they are more active. Even taking all that into account you are missing the biggest difference between them: size. The consumption of energy is not directly related to the size, in fact, the energy requirements drop exponentially with the reduction of size. The same phenomenon explains why they are far stronger, fall-resisting...