So this is what I’d realised. I tried the small 19 in store and it kept falling off my shoulder. I bought it nevertheless as I was fine to carry it by the handle or crossbody. I took it out for the first time and packed it full. Surprisingly due to the weight of the bag, it didn’t slide off my shoulders at all, but it could have also been because of the off shoulder top I was wearing - vs a sleeved top when I tried in store. Under the weight of the bag, the shoulder pad moulded to the shape of my shoulders and I felt like over time it would be shaped to help it to stay on the shoulders even if not packed full. It wasn’t that heavy when I packed it full either, just a bag organizer, a long wallet, earphones, tissue and a light t shirt. When I tried in store, the bag only had my long wallet and phone in it.
I would totally recommend the small as it is roomy enough. I contemplated for a long time between the m/l size and small - glad that I got the small size as it did look just nice when packed full. I could imagine the bigger size looking too bulky for my frame (I’m 5,2) if packed full. If not packed full, then the m/l will likely sag overtime. Packing the small almost full helps to prevent sagging too.
If you don’t like the squished look then a bag organizer is highly recommend. I have one for my bag and it really helps to keep the shape when you are carrying it around or storing it. Samorga is a popular brand for the organizer.