Any ideas why is it that I have a lot of acne on my chest and back and so little on my face?

What I can tell you. You need to do something about this. Acne on the trunk seldom responds well to anything short of systemic prescription medication. It's a serious personal and professional liability that you will need to manage with a scientific physician's help until it self-cures as you get older. Pick up the phone now and schedule an appointment, and begin or continue using topical benzoyl peroxide.