Why are bike frames getting so much bigger
From metallurgy for cyclists:


  a tube's diameter increases (D), the stiffness increases to the third power of that number (d is the inside diameter). Comparing a one-inch tube and a two-inch tube of equal wall thickness., the fatty is going to be eight times as stiff as the little weenie tube. And the weight will only double. Now does the ride of those Kleins and Cannondales start to make sense?


So wider diameter tubes actually allow for less material to be used for the same amount of stiffness, making for a lighter bike with a similar ride.