Actually, if you look at the spec on the Apple website, you'll find the weights for the MacBook family as below:
MacBook Air 1.36kg MacBook Pro 13" 2.04kg MacBook Pro 15" 2.49kg
The weight difference is very little between 13" and 15" model, the difference is about a can of coke.
But the size a bit larger, making it more difficult to lug around.
Even when comparing the weights of MBA and MBP 15", you'll find that the weight difference is still quite insignificant; considering you get larger LCD and built-in DVD drive.