This may be true for plants, but for insects and mammals to grow "giant" requires a high OXYGEN cocentration, not Carbon Dioxide. High CO2 concentrations in mammals induce panic response–this is why you can easily asphyxiate on Nitrogen; your body does not sense it is suffocating in response to a lack of oxygen, but rather, to an excess of CO2. You keep breathing the nitrogen calmly until you pass out from hypoxia.
Insects breathe through "spiracles" which pull air into a tracheal tube running along their whole body, and which is not directly connected to their circulatory system. The trachea exchanges gases directly into their body tissues. This system is inefficient when scaled up, but works OK with higher concentrations of oxygen, such as those found on the surface when the planet was younger.
https:// en.wikipedia.org/wiki/Respiratory_system_of_insects