keep this in mind, a sump could be referred as an un-pressurised canister filter as it basically works the same. water in & out through some other parts to filter it.
The canisters are rated on flow not litres they hold. So as long as the water is flowing through media that filters & maintaines bacteria that cleanse the water then it will do the same job.
if the water flows into the sump, through filter wool it will remove solid matter, then through media like bioballs etc it will then have bacteria taking out what it needs to etc & then flow through to where the pump is to return it back to the tank & so the circuit starts again. Its up to the flow that you want & the pump you use that determines the amount of filtration you get.
at the end of the day, it doesn't need to be to complicated as livingart has shown.
here are some pics of a basic sump for a big tank that also doubles as a fry tank