I've always hated my upper body, after a BBL my back is still a bit large. What do I do to make it smaller? (photo)

Thank you for your question and photo. Without seeing what you looked like before your BBL, and no physical exam, it's tough to say. 
I recommend scheduling an in office consultation with a board certified plastic surgeon for a more comprehensive plan.
Good luck!

If your back has fat, then liposuction may be a reasonable option for treatment.  Best to be seen in person to be properly evaluated.

When undergoing body contouring procedures make sure you surgeon takes into account how changing one area with affect the overall aesthetic and contour of other areas. For example, if you do liposuction only on the lower abdomen, your upper abdomen can look bigger. The same can be true with BBLs. Often, the waist undergoes liposuction to help make the butt look for more and round, but that can also make the back appear wider. 
I recommend seeing a Board Certified Plastic Surgeon to discuss this issue. The fix may be easy with targeted liposuction. Or it could involve excisional body contouring surgery. Either way, I am optimistic that it will be manageable.
I hope this is helpful!