Again, this is due to battery size and charger output. S3 wall charger is 1000 mA, battery 2100 mA. Not sure what iPhone charger is, but probably close to that. Think of charger rating as flowrate of a garden hose, and battery rating as size of kids pool. Smaller pool fills faster with a given flowrate. Then what you're doing with your phone while it's charging, mAh draw, is how much water you're scooping out with a bucket while it's hose is filling. Lots of variables there so it's hard to compare the two without controlled tests.