Zorb is an extremely absorbent fabric developed my Wazoodle, which allows less layers (so a trimmer nappy), but more absorbency. It's cellulose and microfibre based. It's slightly thinner than a layer of cotton fleece, but 5 times a absorbent.
My preference is for the bamboo velour or interlock outer, with bamboo hemp fleece core, bamboo velour or interlock inner, bamboo hemp fleece core inserts and either the zorb or bamboo hemp booster layers.
The fit is similar to the UD nappies as the design stemmed from there, but it's trimmer in shape and layering and the fabric options are very different so that changes the fit really.