The inventory space limit is there to keep the game running at optimal performance. If you were to go too far past that limit, the game would slow down to a drastic level and it would take multiple extra cycles for the game to calculate the next action, because each item in the game takes up a small amount of processing power.
I think the reason your max inventory scales with the number of hearthlings you have is in part because if your computer is able to handle more hearthlings at a time, it would be able to handle a few more items as well, until you reach a point where the game starts acting slow and you stop allowing new hearthlings to join, preventing your item limit from going up and your computer performance getting any slower.
It’s just item management. You can disable the limit completely by editing a file [do a search if you really want to], but it’s highly unrecommended because eventually your computer won’t be able to keep up with the item count.