890 cu inches isn't a lot of capacity. You average backpack holds about 2000. However, it does look to be a little larger than most trunk racks... just don't expect it to cart a change of clothes and a lunch.

I have no idea about the rack. It does have 3 vertical supports which is really nice when trying to keep your bag from rubbing up against a fender stay or even a spoke. However, I don't see how it attaches to the seat stay... they probably dropped that out of the photo.