What are all the factors that increase the chances of crafting double resources?

Solution 1:

The two biggest things that contribute to bonus crafting/refining:

• Your respective skill level
  • The higher your level, the better chances at bonus items you get.
  • Being 200 in the respective skill will grant you the most bonus and each trade skill will provide a specific bonus found when you click on the trade skill. E.g:

• The reagent used in the craft
  • Using Tier V reagents in the craft/refining vs Tier III will increase your yield.
  • In the same vein as using higher tier reagents, you'll get more bonus materials when refining lower tier stuff. Not sure what the exact % is, but you'll notice you will get a larger amount of bonus materials when smelting iron vs orichalcum for example.