I think the limiting factor in this is not how much can you cram into a backpack, it’s how much can you carry. I had 2 US Army backpacks, one for me and one for my wife. I packed them up with things I might need to survive and had to stop when mine weighed 80 pounds (36.3 kg for those on the metric). My wife, being much smaller, had to stop at about 30–40 pounds
When we decided to pack, we were picturing having to live outside for an extended period of time. Things we packed were:
Tent with ground cloth (tarp)
sleeping bags
fire starting materials
rope
knives
compass
compressed/dried food
water purification stuff and canteens
books on foraging and plant identification
tools that might help us such as a survival shovel and a Leatherman tool
stuff to cook food – camp cook pot, cutlery, etc
Medicines my wife needed
A well stocked first aid kit
something to repel mosquitos and ticks as well as treating bites.
Something to do for entertainment – pad and paper, books, dice.
Extra socks and underwear
two changes of clothes and some way to wash what we had.
Toilet paper. We have since upgraded this to compressed towels (Discs about the size of 4 quarters stacked that, when you add water, expand to be a towel about the size of a washcloth.)
Primitive fishing equipment
Money (cash, silver and gold)
Some things like a rain jacket or winter wear we would just wrap around our waists so as to not take up space in the pack.
That was a big load. Some things are bulky but not heavy, others are heavy but no bulky, some are both. It wouldn’t be enough to live indefinitely but it would be enough to survive for quite a while.