I have the DT3. The duplex system is awesome. I'm not sure wht the bag size is, though. Kifaru's website has the entire bag at 3700 or 4000, depending on which spec sheet shows up. I think the external pockets are around 1500-1600. Based on those numbers, the main compartment could be anywhere between 2100and 2500. The main compartment holds a quite a bit of gear. If I were to redesign the bag, I would make the mouth larger. It's difficult to squeeze bulky items into the toploading pack because you lose some size because the side attached to the duplex frame is flat. This makes the mouth narrow. Imaging a circle that has been flattened on one side.