i think youre confusing media volume with available surface area for bacterial colonization. with a good quality bio-media (such as the bio max pellets the AC20 come with) you get a ton more surface area than other media like ceramic rings (i believe its 6-10x. i dont remember the exact amount, id have to look it up). so even though the volume of the media basket doesnt look like much, there is a lot more room for microscopic bacteria than you can see.
also with a properly stocked 20g tank, the 6x turnover of the AC20 is more than enough. however, i agree with more filtration = BETTER. smallest filter i run on a 20g is an AC50.