Q: Why do I need to lean out my carb when I climb?

Stealth Pilot is rude but he is also correct.
I think your confusing volume with density. As the aircraft climbs into less dense air the amount of oxygen for a given volume decreases. That IO 360 engine (ie) is still breathing in 360 cubic inches of air every 2 revolutions, but that air is less dense and carries less Ox at altitude than it does down low. A rule of thumb is that 50% of the Ox in the atmosphere is below 18k'.