In fact the opposite is true. Because you are constantly picking up your iPhone, the weight is constantly apparent. But because a watch is strapped to your wrist and becomes a part of your arm you wouldn't even notice an extra ounce. I mean, even if you have a chunky watch on, does your arm ever feel like it is weighted? Then there is the leverage factor to consider, something strapped to your wrist is much closer to the elbow fulcrum point than when it is held in your hand.