The shape of the top case is also important to consider.
These two boxes are used on my scooter:
The Givi is 55L, and yet, it cannot hold my two full face Arai helmets w/ Sena Comms. It's a
clam shell design, with curved surfaces, and Givi makes them double wall construction, which takes away from cargo space:
This Givi is 58L, but w/ parallel sides, it seems like I can store 30-40% more cargo than the E55 above. I can easily swallow the same two Arai helmets, and they are sitting upright, not on its side which can damage the Sena Comms.