I use a Nelson Longflap, and take advantage of the capacity to carry whatever I feel I might want, and also leave stuff in that I might have put in previously but haven't got round to removing.
Typically, something like...
HPx, 3xtubes, tyre boots, steel levers, 2 multitools, some chain oil, pair of brake pads, quicklinks, waterproof jacket, waterproof shorts, waterproof cap, sun cap, legwarmers, gloves, spare bike lights, headtorch, 4xAA for the GPS, 2 or 3 muesli bars, plus whatever else might have found its way in.
Last time I weighed it, the answer was 4 kg.
Talking of CO2 cartridges. I often see loads of what look like CO2 cartridges spilt by the side of the road.
Are they actually shed from cyclists who haven't zipped their saddlebags up tightly enough or are they actually laughing gas or something from the local yoof?
There's normally one or two but yesterday I saw dozens all in one place, which suggests they're not from a single cyclist.
NO2 cartridges, as spotted here:- https://yacf.co.uk/forum/index.php?topic=53164.msg2470948#msg2470948
N
2O
Any user who tried using NO
2 instead would end up dead