I use a 48L and always have space to spare. OK, I don't take cooking equipment (I eat out) with me but if I did those items would go in easily. I have used my 48L rucksack on 2, 3, and 4-week hiking trips and have always had more than enough space. I also don't carry a camera, just my smartphone.
One reason for this, of course, is that I have actively tried to make my load lighter and smaller in overall size and so now only take what I need, rather than what I THINK I need. I also have some really good items that roll up very small, like my Thermarest Neoair mattress (to the size of a Coca Cola can) for example and I also try to develop the idea of dual-use for clothing, for example when I sleep I wear a clean and dry base layer and will hike in it the next day, and will use my single skin waterproof coat as a wind jacket as well. Hiking like this with the equipment and clothing that I now use has helped me to make very useful progress for myself. I have also deliberately bought lightweight items, a down sleeping quilt rather than use a (heavier?) down sleeping bag and I use a very lightweight rain skirt rather than use (heavier?) waterproof trousers. I've learned that as I get older, every little step forward helps me a lot!
My way of needing less rucksack space is that I also carry items on the outside of my rucksack, my tent, for example, that just sits on the top of my rucksack horizontally, held by two short lengths of elastic.