Weight in general seems to be broken. The weights of the items don't add up to the total weight my character is carrying.
OTOH there doesn't seem to be a penalty for carrying wayy wayy too much.
Also backpacks don't seem to make any difference (well, except for being able to more easily organize by category).
What I did with backpacks is put it on my hotbar and fill it full of potions and scrolls. I click on it and select what I want to use easily. (Gotta remember to right click and use, otherwise you send it to your inventory S looting)
If your lift is 75 lbs you can carry 150 lbs of stuff. Its not encumbered till after you exceed the carry limit. (Yes its confusing)