Are low concentration's of CO2 produced naturally in the aquarium through biological activity in/at substrate (organic carbon), and also as by product of fish respiration.
In low tech tank's ,you want to try and keep it from out gassing too quickly as would happen with water fall effect from HOB filter or sponge filter's.
Not so much of an issue in CO2 injected tank's where one can increase the CO2 fairly easy.
Yes,canister's will hold more biological media but only what is needed for amount of food/waste available from fishes and feeding's.
Bacterial colony that processes the waste is found not only in the filter, but on all hard surfaces in the tank.The glass,the substrate,the wood,rock's,plant leaves,inside hoses of filter's and inside wall's of filter.
Bacterial colony grow's to what is needed to deal with the waste, but it can easily be overwhelmed by sudden addition of more fish ,and more food usually offered with additional fishes.
Plant's are good biological filter's also, as they can use the ammonia,nitrites,nitrates, for growth much faster than bacteria it is said.
Believe is why most use mainly pad's ,sponges in their filter's with good plant mass.