At a whopping 16L, this is the biggest bag you can fit under your seat without a rack. For a smaller version, check out the Dropper Seatpack.
It is meant for light, large volume items like a sleeping bag, tarp, and clothes. This bag feels solid even loaded up on the bumpy stuff because it has a 360° internal plastic frame that holds its shape. Rugged metal cam buckles means secure, no-slip connection points. Durable, water resistant construction. Good thigh clearance with room to get behind the saddle. This is a no-frills work horse with one strap for compression and extra gear.
*Must have 4.5" of exposed seatpost and 8" saddle-to-tire clearance