To a large extent it also depends on where and when. My winter sleeping bag takes up over 20 litres... On the other hand, when I did my military service we had the LK35, with a crappy sleeping bag, a foam pad on the outside, a parka, spare socks and a cook kit, food for 24 h. In winter. But mostly we slept in tents, and somewhere the was a duffel bag with spare clothes, etc.
For a summer overnight you need:
Night time kit: sleeping bag, mat, tarp/hammock. Or just a blanket. Where, when and what skill- and comfort level?
Cook kit: one pot, one spoon if you are solo, perhaps a stove rather than an open fire pot (is there a fire ban?). Kuksa as a luxuary item? I often carry a small piece of scouring pad (i.e. the same as the scourer side on the sponges), but some Equisetum will do just as well.
Food: no need to get fancy, a salami, a slab of bacon, some noodles, pasta or rice, perhaps a small onion, spices if you want them. Tea can be carried or made from fresh plants (a small "fabric" filter is nice). Water is an issue in some places, I almost never carry a canteen in the northern half of Sweden, seldom in the rest either as a matter of fact. Some places a waterfilter is essential unless you ant to boil all water (and then a canteen becomes a virtual nessesity)
Clothes: You'll be wearing most of them, but spare socks, rainwear, an extra wool shirt, etc might be nice; it depends on what you may encounter and your standards. Mosquito lotion and head-net is handy in some places and seasons. Toss in a couple of FC sticks in pockets and pack, and fire is almost guaranteed. Gloves are nice in the north, mostly due to mosquitos. Hat of some sort (beanie, felt, waxed cotton)?
Miscellaneous This is the section that can grow without bounds; don't let it do so. A belt knife, a small FAK, perhaps a small torch, binoculars, etc almost allways come in handy. But a bandana or a washcloth can do for towel, no need for a shave, a small bar of soap might be nice. I think it was Mungo Park who "carried" two books per day (i.e. his porters carried them), no need to emulate all the habits of the 19th C explorers, but one book can be nice.