I've seen multiple reports in forums that the Strong Back ability does indeed increase your carrying capacity from 250 to 300.
What I have not been able to confirm is how you actually obtain this ability. Right now it appears to be a mystery among the gaming community. Some people have it, and some don't, and nobody can explain exactly why. I saw one post on gamefaqs from a guy who got it in his first play-through, had to restart after wiping his hard drive for an unrelated reason, and now in his new game he doesn't have it and is clueless why.
This is currently my biggest gripe about this game. I left the prologue and entered Ch.1 carrying 437 pounds of stuff. This includes 70+ pieces of leather and 41 pieces of iron ore. Even after selling every extra armor and weapon I had picked up (including a couple I would've liked to keep around as backups), I still only got down to 265 (and like the original poster in this thread, I want to keep all of my crafting supplies for later).
Managing the inventory weight is clearly going to be a big part of my game, and I am very frustrated that something so fundamentally useful has been relegated to an undocumented perk that is apparently VERY easy to miss (as I have). I can understand stuff like +10% protection or +20% backstap damage getting rewarded this way, but I think it's bad design to randomly screw around with the inventory management like this. When I go back to play this after work tonight, I'm probably going to go back to an earlier save in the dungeon and try to luck into getting this (and will be very annoyed at doing so).
It's really irksome that the official forms are down so we can't troll for more info there. Also irksome is that the "game guide" that I got with the digitial premium edition doesn't talk about ANY of these special abilities.
Anyway, if anybody finds anything more about this or other abilities, please share your findings here.