The largest capacity will be something that uses thin material and has hardly any features. If the Landor & Hawa Sub0-G bag exists in a 20" size I imagine that would be it. The Travelite bags might be a close second. They are even better for weight but do have more features (I know that's strange).

Otherwise you could get something like an A. Saks 21 bag and use a cart with it. Or an Airboss and a cart.

Duffel bags often have higher volume than "normal" suitcase shape bags.
20x14x9=2520 vs. 20x12x11=2640. Both would be 43 linear inches. This obviously also translates to rolling duffles.

Till