Stockpiles should have the benefit of holding more items than a cupboard, with the disadvantage of being unable to see the quality of all items inside.
Some stockpiles already work that way, and are the most useful: blocks of wood, boards, stone, and so on... You only put them in cupboards if seeing their quality is a priority for you, otherwise you use stockpiles to fit more in a smaller space.
Unfortunately there are tons of stockpiles that don't work that way. They hold less items than a cupboard would, while keeping the original disadvantage of not letting you see the quality. All fruits stockpiles, cattails, bars of metal, petrified seashells, flowers and so on.
Someone could argue that stockpiles have the advantages of being placed anywhere, compared to cupboards. Unfortunately there's plenty of stockpiles that are still smaller than a chest (which is pretty cheap), holding 25 or 35 [1-slot] items only.
There are a few stockpiles that fall in between: fish is one of them; great for [3-slot] large fish, which holds the equivalent of 90 slots. Decent for [2-slot] medium fish, holding barely less than a cupboard. Awful for [1-slot] small fish, holding even less than what a chest would, in twice the floor space. Hides are another example of this.
That's why I think small stockpiles should be changed. Ideally, any stockpile should hold more of that specific item than it would fit in a cupboard. This would allow players to choose between convenience (seeing the quality by putting in a cupboard) and space saving (by using stockpiles). Also stockpiles look super good and I want to have a reason to use them!