Your back muscles may be getting bigger, though not necessarily. A recent study showed that patients can have improved strength and endurance without having major changes at the muscular level. Patients in this study did better on physical tests after three months of regular exercise therapy, though their muscle size and type had barely changed at all. These improvements are most likely due to better muscle action and coordination.
Researchers suspect that it takes more than a few months to reverse the process of muscle shrinkage that often goes along with low back pain. This is especially true if you've been inactive for a while due to back pain. The longer you stick with your exercise program, the more likely you are to "grow" your muscles back to their top condition.