The bigger tire, the harder it is to spin, therefore less power, but if you have the power it will give you more top end.
Think about it in terms of real trucks. See how slow and hard it is for the lifted trucks to take off while a stock truck of the same model will smoke it off the line. Once the lifted trucks get moving they will have a higher top speed at the same rpm as the stock truck.
So as you can see, you don't ever get more power, you just gain in how you want ot use it. Smaller tire, quicker out of the hole, bigger tire, more top end. But be careful sometimes the bigger tire will hurt you if you are trying to haul a$$ up a big hill, if your quad doesn't have enough power to turn them.
Hope this helps.