Alright, lets say a standard worker starts with a backpack containing 4 item slots.
At level 1, a Weaver can create an expansion that allows for a total of 6 items, adding 2 to the max total.
At level 3, he can now add 4 more slots, for a total of 10 items.
Finally, at level 6, he can add one last expansion of 6 slots, for a grand total of 16 items to be carried.
Of course, we’d probably have to do more than add a level gate. Suppose a Weaver needs sturdier cloth, for example, to craft the second expansion at level 3. Perhaps for the third and final expansion a level 4 Blacksmith would need to create an expansion out of some type of ingots, like a chainmail sack or something.
This is to prevent workers carrying too many items too early in the game. Of course, it would have to be tweaked, perhaps lower the level requirements for the Blacksmith or something. Then again, what’s to prevent someone from crafting the expansions out of order?
We also have to take into consideration the fact that Workers will rarely need to carry around 16 items, especially considering what’s in the game is mostly early-game stuff. Once the game gets into mid- or late-game I could see 16 item backpacks being useful, but for right now I think they’d slightly overpowered.