It's confusing. Topeak makes several variations on the same bag, or perhaps there are different models of the same bags from different years floating around. Add to that that I think they have or had two separate quick release track systems and that some bags can be bought with no quick release system.

The one you linked still does not seem to be the biggest one because it does not have an expanding trunk area. That's a feature I enjoy because while I love the option of the fold out panniers, I do find that I prefer it if I can get everything into the main trunk area. But I do like having as much storage potential as possible, and I do occasionally make use of both the fold out panniers and the expanding trunk. If it's more storage you want, take some time and make sure you get the largest bag possible.

But the Topeak bag does have some heft to it. I'm surprised how much lighter my bike feels when I ride it without the bag. For that reason, you may also enjoy a smaller bag for times when that's all you need. If your budget allows, you may find that you have use for both bags. Right now I either have a giant bag or no bag at all. Somehow even though I carefully consider whether or not I might need it on every trip, there are many times that I leave the bag at home and find myself wishing for it. Perhaps a smaller bag would solve that problem by giving me a lighter weight insurance policy for trips where I don't plan on carrying anything home but end up with unexpected baggage.