0.2l (that’s 2/3 of a coke can, it’s tiny!) saddle bag from decathlon takes a CO2 inflator, tube, multitool, patches (rather pointless as I don’t cary a pump most rides!) and chain link.
That’s it for short rides. Longer rides a 2nd tube in a jersey pocket and a frame mounted pump (usualy a propper one, not a fiddly little ‘mini-pump’).
CC, £20, phone and house key go in a sandwich bag in the left pocket.
That leaves the other 2 free for clothing in the middle and ‘food’ in the right.
As for water, I used to follow the ‘mor eis better’ approach and take 2 750ml bottles on a 15 mile comute! But I think it’s an MTB’er thing to drink way too much as it’s too easy with a camelpack. Drink enough before a ride (including the 48 hours before), a couple of pints of water before bed and another pint of water with breakfast and another imediately before leaving the door is enough to keep you going for a very long time without pissing. If I get through more than one 750ml bottle now it’s a rarity, even on a really hot day 2 bottles will do me for 60-80 miles. How many piss stops do mtb’ers take, I’ve never stoped on a roadie group ride for someoen to piss.
I think there’s a parralel with nutrition, people spend ages planning their pre-ride meals, but don’t think to drink anywhere near enough. If you’re 100% hydrated and another pint sloshing through your gut you’ve got a good 1.5-2kg of water to sweat out before it begins to afect performance.