What I carry in it is:-

• sleeping bag (YHA Plume 2lb 13oz mountain bag, now over 30 years old)
• old fashioned over-your-head canvas anorak
• cagoule & waterproof overtrousers
• spare gym trousers for walking
• thick wooly jumper
• decent slacks for hotel use
• 2 decent shirts ditto
• plimsolls ditto
• pair of socks & underpants (I wash them as I go along)
• spare walking shirt
• ski gloves
• YHA wooly balaclava hat
• 2 x 1 litre plastic water bottles, full to start with
• dried fruit or sultanas for lunches
• Kendal mint cake (®Wilsons)
• emergency small tin of corned beef
• sponge bag with:-
  • toothbrush & paste
  • soap (2 small hotel pieces, in case I lose one, easy to do in a hut)
  • flannel
  • footrot cream (Mycota)
  • Savlon (®ICI, antiseptic cream)
  • sun cream
  • sun lipstick
  • hairbrush
  • nail file
  • 'housewife'
  • scissors
  • sticking plaster strip
• old-vitamin-pill-tube of detergent (as I wash clothes as I go along)
• camera
• camera battery charger
• maps, compass & whistle
• paperback for evening reading
• tiny waterproof diary & pen
• (usually) day sack

On the outside is an ice axe, sometimes (as in 2001) crampons & a light ladies brolly, usually used as a parasol but occasionally as a brolly.

For a day sack, I use a cheap Swiss tote bag with a couple of strings, so it can be carried as a rucksack. This is about 5 litres. It only has to carry a water bottle, camera, map, compass & lunch. This is very small: nothing like your suggested 25-40 litres.

I also have a 75 litre rucksack for when I take my more elderly wife to the hills, so I can carry everything apart from the camera, food & water. She carries these in a Karrimor 'Vista 18' (18 litre?) day sack, which is really bigger than necessary. The 75 litre rucksack is also used to raid the local supermarket every week.

Note...that, no matter how beautiful the weather may appear to have settled to,
I am prepared to meet a sudden blizzard. A cloudless day in the Alps can change its mind!