I did a few tests with Blacksmithing. When you craft items, you can choose how much of the raw material to use. As you add more material, the level of the item goes up. Also, the "experience points" you get toward the skill go up. However, what I found was that the gains didn't depend on the base amount of material used.
Example: A greatsword takes a minimum of 5 bars to make. You get 48 xp for making one, and if you deconstruct it you get an additional 13, for a total of 61. (Note: I couldn't test what you got for de'ing someone elses, it is supposed to be more.)
A dagger takes a minimum of 2 bars. It gives exactly the same xp gain (48 for crafting, 13 for de-crafting).
The best result was to use the most possible bars on the cheapest possible item...if I craft a dagger with 8 bars I got a total of 228+65 = 293 points. The greatsword at max level required 11 bars for the same number of points.
So, if you aren't trading materials around, use the most material you can on the item with the cheapest base cost.
I didn't have enough cases to track the amount of material you got back on a deconstruct, but it seemed like the higher level weapons returned more.