My 15" MBP slides easily into a thin pocket along the back of my backpack, while my 17" Alienware m9750 takes up the entirety of the main pocket. Putting both into the backpack makes it very heavy, and uses up almost all of the space in the backpack.
With my old backpack, again, the MBP slides easily into a thin pocket along the back of the backpack. However, the m9750 cannot even fit in the main pocket, the laptop is just too big.
It has nothing to do with how much you can hold at once, but how much you want to deal with all day, in all sorts of situations. Once I neglected to grab a cart at the grocery store because I figured I could manage four gallons of milk, which I could (mind you I'm pretty scrawny); however, I realized it was not only a stupid idea, but a very stupid idea because I had forgotten that I had to go grab something else which was at the opposite end of the store.
Yeah cool I can lift four gallons of milk hooray, but if I plan on doing anything while carrying those four gallons of milk I want a cart.
It's the same thing with laptops. A 17" 10lb desktop replacement is going to be a giant pain to move around, a giant pain to fit into things, and a giant pain to work portably with.
With all this being said, a lot of people are fine with the reduced portability, but it's just simply not true that they're just as portable as any 13"/15" Ultrabook.