There seems to be a lot of confusion as to what people are referring to, when they say backpack. You can get backpacks big enough to carry a whole camping trip worth of supplies, or small like a schoolkid's backpack for their books. There are carryon-sized soft bags with straps that let you carry them on your back. You can see Rick's versions them on the website. And then there are small ones like the Civita and Ravenna backpacks that people use as personal items both on and off the plane, that I think more people refer to as "daypacks".
So for example, look at the Rick Steves, luggage page, and see the Classic Backdoor bag (a backpack because of the straps and no wheels) which is my preferred carryon. I use the Civita as my personal under-the-seat item.