Temperatures, lighting, and lack of exercise makes the difference between what indoor and outdoor cats experience. Indoor formulas (at least, Nutro's indoor, I don't know about others) have antioxidants for the immune system, more linoleic acid for the coat, Taurine, DHA for the nervous system/vision, FiberClean to help pass hairballs through the system, OdorCheck to reduce stool odor, Advanced Dental Care for clean teeth. Easy to digest, less shedding, fewer hairballs, low magnesium (max .085%).
I typed most of that from the booklet they put out for the Nutro Complete Care foods. I'm switching my fatty cats to the Indoor Weight Management, since my efforts to reduce Gizmo's weight let her gain a pound!