Oxygen uptake with gills from water vs lungs from atmosphere

The concentration of oxygen in water tends to be around 6.5-8 mg/L. In the atmosphere, it is 21%, one liter of air weighs roughly 1.25 gram, so, oxygen concentration is 0.25 g/L or 250 mg/L. So, lungs can access 50x higher concentration of oxygen than gills can. This suggests, sort of, that maximum oxygen uptake possible should have a significant increase when lungs evolved. Is that trend observed and a topic that is often talked about?