My daughter uses a backpack as a carryon, and recently Frontier said it was too big for a ‘personal item’ and charged her and her BF $100 at the gate! (that is why I don’t fly Frontier). A lot of the European airlines will only allow bag the ‘fit in the box’ (HxWxL) and those boxes are smaller than what the US carriers allow.
I told her I would have put on every item in the backpack and wore it onto the plane, looking like an Oompa Loompa but saving $100.