If carbon dioxide makes up less than 1% of the atmosphere; oxygen constitutes almost 20%?

Question

Why the difference, since both are used in the carbon cycle?

Answer 1

The difference is because of historic accumulation:

photosynthesis came before respiration, so O2 producers had time to increase O2 levels via photosynthesis (decreasing CO2) in the atmosphere before respiration of oxygen by organism even exhisted.

as O2 users started using, O2 producers had the upper hand in production in the historic record, so oxygen accumulation continued in the past, producing higher and higher levels of oxygen, peak of ~30%.

There is presently a dynamic equilibrium (production and consumption occur at the ~same rate) of these variables that keeps these levels at ~20% O2 and <0.05% CO2, but these levels were set by historic accumulation.

Answer 2

Because Oxygen is a product of photosynthesis and Carbon dioxide is the fuel. Oxygen is the fuel for breathing (but not as much as carbon dioxide needed for photosynthesis) and carbon dioxide is the product.  Also some of the water molecules seperate but that makes up for only small percentages.