It would be, but if you have a full one of those, plus large wheelies, you're getting ever closer to the edge, particularly at Flight Connections. So either use the smallest of these backpacks you can get, or a large one and don't do wheelies. As I say, BA isn't wildly fixated on this, but if you come lumbering through Flight Connections with the kitchen sink, it may just coincide with the twice a year crackdown. Plus the not inconsiderable issue of courtesy to your fellow travellers.