I cant remember where the topic is but i dpo remember someone saying tht in a sertin quest you get a backpack that tahes up like 4 aquares and gives you 20 more squares of available space.
A quest in perticular, but i have not seen the subjet again since i first read it on the forums.
Unless i have mistaken it fpor just finding the backpack for one of the first secondary quests in broken world.