i don't think that is intirely true; i use co2 diffusion in my tanks to
increase the growth rate of plants. it's similar to an air bubbler. co2
is pushed into a tube which then forms bubbles. instead of rising
directly to the surface, they go along a plastic track which slows down
their ascent. as the bubbles rise, they diffuse into the water and
become smaller. co2 becomes measurable in the tank and it drops the ph
by 0.5 to 1 whole ph.

with air diffusion, i imagine that the bubbles do decrease in size to
some extent. since air is free and pletiful, their's no need to work
hard at diffusing it. just pump all you can in there and some will
diffuse. surface aggitation alone cannot get enough air in a tank if
there are many fish.