Cactus Jack Field Expanding Backpack has a main compartment that is 19 x 13" and it expands from 5" to 10" to accommodate extra gear.
 
Main area expands from 5" to 10" to fit more gear
2 accessory pockets
Water bottle mesh pocket
2 electronics pockets on shoulder straps
Removable waist belt
Daisy chain holds rain gear, etc.
Up to 2,470-cu. in. capacity, 40.48-liter capacity