This bag has a sleek design that suggests it has a couple of tricks up its sleeves. It doesn't have sleeves, but that's beside the point. It has the tricks.
It has an unexpanded capacity of 900 cubic inches, including the two side pockets for easy access to a camera or other smallish gear. But when you expand the main compartment, you have a total of 925 cubic inches for food, clothes, and way more gear than a rack-top bag is expected to carry.
Attach a safety light to the web loop to supplement the 360 degrees of reflective accents.
For an expanding rack-top bag that starts and expands even bigger, go for the Banjo Brothers Rack Top Pannier Bag.
Banjo Brothers Quick-Release Waterproof Handlebar Bag
Thule Tour Rack
Thule Side Frames