When looking over the rules of a few O D&D retroclones, I noticed a trend: most backpacks are only listed as being able to carry 30 or 40 pounds. From what I've tried to look up though, people can carry far more in a backpack than this, even though carrying that much weight on their back might not necessarily be good for them. Usually, the backpack is in less danger of breaking than the person. So my question is this: What is a more realistic estimate of the weight a backpack can safely hold before being in danger of breaking?
First some data to get a handle on how much people carrying.
I found this excerpt on google books where a newspaper stated that a porter was expected to carry 100 lbs for ten miles a day.
In Andes regulations limit the weight porters carry to 20 kg or 44 lbs.
The general recommendation is 20% to 25% of your body weight if you want to hike all day. Which works about 30 to 40 lbs for an average person.
You question was how much weight can a backpack carry? Well D&D doesn't specify that specific construction of the backpack. People had backpacks throughout the age. Mostly made of hides or leather. Like today, how much they can carry depends on how they were constructed.
Because of the various recommendations on how much you should carry while hiking the D&D rules on the size of backpacks seems reasonable. Likely were developed in light of the experience of the developers. You could carry up to a hundred pounds on a frame designed for that purpose but you would limited to 10 miles a day or so.
Probably looking similar to this guy