pbusardo measured in it his review. The RBA vs OCC does NOT change the capacity with the mini tank. This only happen with the full size Subtank. He was able to get 4.2 ml of e-liquid into the mini subtank. Kanger advertises 4.5 ml. That 4.5 ml may be filling it, letting it soak into the wick, and then refilling it.
Regardless, 4.2 ml will completely fill an empty tank with either the RBA or the OCC atomizer.