What is the actual size of a 26x1.5/2.2 tube?
Tubes expand quite a lot as they are inflated - Try pumping one up on its own and see how quickly if expands (don't over do it unless you never want to use the tube again.). The way tubes and tires work, the tube holds the air in, the tire prevents the tube expanding.  

The 1.5-2.2 means the tube is suitable for a tire 1.5 up to 2.2 and is ideal for you 2.1 tire.