Hi,
as far as I know, the gained skill xp are related to the value of the item you create with the recipe. So if you want to decrease the gained skill xp, you have to decrease the value of the crafted item.
Alternatively: You can decrease the gained skill xp amount of every (!) smithing action, if you go (in the CK) to Character -> Actor Values -> Smithing and decrease the "Skill Use Mult".