1) Better thermal performance so it'll run faster
2) Bigger screen
3) Better graphics card so photo and video editing will be faster
The 16" model is a little bit bigger, but it isn't as simple as that. Since it is bigger, not only does it get a larger screen, it gets better thermal performance since the cooling fans have more room to work with. This also has a better graphics card meaning it can render photos and videos much quicker aside from the better thermal performance. If you have a bestbuy account you can get the 16" for $100 cheaper than the smaller one.