The Dual is small enough to nestle under a seat without scraping against your thigh, yet it's big enough to hold two 700C tubes and two CO2 cartridges, plus an inflator. So, why two spare tubes? Because you never know what's up the road, and it's nice to know that you're prepared. That extra tube will make the rest of your ride that much more enjoyable knowing that you won't have to use your cell phone if another flat occurs.
27 cubic in.