My purse is fairly big - it also contains my laptop computer (but it's a purse, not a computer bag). I fit both the purse and camera bag under the seat in front of me.

I need to carry all 3 things (camera bag, computer bag/purse, carryon bag) with me. My laptop is just barely too big to fit into the camera bag, and I don't want to use a larger camera bag because this one is big enough already. Anything bigger is too uncomfortable for the primary job (camera bag) when I'm at my destination. I just downsized from a larger camera bag to this one, and it's perfect for my camera gear.