A 20-lb CO2 cylinder is about 8" in diameter and 27-1/2" in height. I think you measured the circumference, which would be ~ 25" if you multiply the diameter x pi. A CO2 cylinder is rated in pounds because when filled, the gas is primarily in a liquid state.
A 22 cubic ft. aluminum nitrogen cylinder is 5.5" in diameter and 18.5" Height. This would give you a ~ 17" circumference. This is roughly equivalent to a 10-lb CO2 cylinder in volume.
Sounds like you bought a 20-something cubic ft. nitrogen cylinder. Is yours steel or aluminum?
It really gets confusing when you are mixing CO2 cylinders rated in pounds and specialty gas cylinders rated in cubic feet.
I just went through this with my local welding supply company. They only do exchanges for beer gas, so I bought a 40 cubic foot steel cylinder from them and swap them out. No one in my area fills beer gas while you wait, so purchasing a shiny new aluminum nitrogen cylinder would have been a waste for me.