If anything lighter may slow the fall and cause less damage.
The Galaxy S II weighs 116 grams. The iPhone 4S weighs 140 grams. That makes the iPhone 4S about 21% more massive than the Galaxy S II. The iPhone 4S doesn't really weight that much more than the Galaxy S II, it just happens to be a lot more dense.
They'll both fall at the same speed, though. Neither object has sufficient mass to cause increased acceleration towards Earth. If the iPhone 4S was as massive as a relatively large object (such as the moon), then you could argue that it would fall faster.
The iPhone and Galaxy S II should both take about 0.43 seconds to hit the ground from three feet, and 0.56 seconds to hit the ground from five feet.
Basically, this all says that there's no excuse for the iPhone 4S breaking so easily aside from mediocre build quality.