Just have at least a stove and pot combined in your inventory. It will now take up 2 vertical slots and the icon reflects this. Play for a while, change servers etc. then open your inventory and "Move" the stove+pot. You'll notice it still has a 2 slot icon but requires 4 free slots to store. Taking the stove+pot apart and putting them back together switches it back to only needing 2 slots.