i don't know about that size 2.55 but my Jumbo classic flap fits an amazing amount..................i have heard that the classic flap fits more than the 2.55, because it stretches more i guess
so maybe the classic medium/large would fit more than the 2.55 of the same size??
i can get as much in my classic jumbo, minus a big sized snack like a turkey drumstick and a pair of flip-flops, as i can get in my GST! i think the only difference is 2" in width
hope this helps, even if i'm not talking about the exact bag, the similarities are very strong