The other thing is a little unfair, but here it is: It depends on how big *you* are.

A large man who can carry the backpack easily, making it look light and small-ish compared to the size of his back, is likely to get away with it without too much of a second glance by check-in or gate agents.

A small 5'1" woman like myself, on the other hand, would probably be forced to check it in. Even though the bag is the same size, it looks huge compared to the size of my back and shoulders. And of course, it would just *look* heavier as I'd likely struggle with it a little bit more.

It might be unfair and a bit of an optical illusion, but there you have it. To be perfectly fair, though, as a smaller woman, my clothes are smaller, too. And I also would be more inclined to check in a 46L bag anyway, since it would be less comfortable for me to carry around an airport.

Pick a carry-on bag that looks light and easy to carry, and chances are nobody will question it.