That's a good point - the stat potions don't have the "Combine with" menu, so it probably means they can't be upgraded. Now, that doesn't necessarily mean there are no recipes for the medium and large ones, we just know that minor ones are not part of the recipe.
It's true, even +3 mediums make no noticeable difference.
So, assuming that mediums or large cannot be crafted at all, I would ask the developer this: If we're not meant to craft stat potions, why give the player ability to craft ANY stat potions at all? Why ONLY minor ones? What is the thinking behind this, if any? Is it just another overlooked aspect of crafting?
We can upgrade HP and armor potions, why not these?
All this time I've been collecting ingredients to make minor stat potions, with plans to upgrade them later when I find recipe. But that plan was a bust.
Sorry but this is BS! It was disappointing enough to find out that crafted weapons and armor cannot be enhanced or sold for any money, and now this. What are you people doing to crafting?????