There are three systems in play.
1 - if you get a line of gems, a mana surge will double the mana collected.
2 - any pluses via traits or banner is applied after the doubling.
3 - for a huge mass of skulls, it is broken into the largest possible single match, and then every additional straight match is calculated separately. That means additional surge, bonus, etc.
So depending on the shape, a mass of 10 gems could be getting either 10 or 20 mana, and could be getting bonus mana up to 3 times.