Even today, CO2 represents just 1/2,375th of the atmosphere.

Before the industrial revolution it was 1/3,571th

That's a change of less than 1/7,000th of the atmosphere.

Oxygen represents 1/5th of the atmosphere.

Some of that oxygen will have been consumed to make CO2, but not enough to matter, and more will have been emitted because CO2 is a limiting factor for plants.

The atmosphere has gotten very slightly thicker.