Hey Lo, are you sure you're looking at the right bag? There is no hip belt, there's a chest strap that you can use (and I did, despite feeling dorky, as it just made the bag more comfortable). It's actually smaller than most European Airline regulations, provide you don't stuff it excessively - I was going to get the regular, as opposed to the junior, but I read reviews and on here that if you're short (I'm 5'3) that the junior is less awkward and I'm glad I went with it.
I did use the compression zipper when boarding planes and the bag was slightly overstuffed using the expansion zipper when we weren't and I could be less worried about size. The trip was EuroWings flight from Stuttgart to Bilbao (no bag issues), rental car to San Sebastian for 3 nights, drove to Polientes 1 night, drove to Llanes 2 nights, drove to Leon for 1 night (dropped off rental car), train to Madrid for 3 nights (including that walk from metro station to my friend's flat - seriously, what was she thinking taking us that far with our luggage as opposed to taking a bus?) then flight to Mallorca, layover, flight back to Stuttgart.
It's not the same as a trip conducted entirely via public transport, as we had a car for a week, but I spent plenty of time on the train/metros/buses with the bag and I was really happy with my choice. Of course my bag was never actually weighed at the airports but it was 7 kg every time just in case. (Ok, maybe 7.1 - what can I say, I live dangerously!) I had to walk with the bag plenty, about a 20 minute walk from the car rental drop off to our hotel in Leon for example, and was again glad I wasn't dragging a wheeled bag through the old city.
I actually plan on using it as my carry-on for an upcoming visit home to California, and will be checking a mostly empty large suitcase to fill up with goods from Our Lady of Target and the Sacred Heart of Vacaville Outlet Gap for my back to Stuttgart.
My only mild complaint it that the bag has probably one or two many small zippered pouches that add unnecessarily to the weight of the bag. I used one of them a lot but there were two others I didn't really use at all. But I didn't see another bag at that price point with good reviews that had a more simple or lightweight design. I am very impressed with the durability and construction of the thing - it's very sturdy. I thought I might carry it like a suitcase given the handle but I packed it so full it was less comfortable than simply carrying it as a backpack. For shorter weekend trips I might use it this way, though.