At risk of contradicting a few on here ....
Too large a fin (when planing) can definitely cause you to head up to wind.
As speed increases so does lift of fin (not lift against gravity but lift against leeward action;ie upwind)
If for a given speed that lift is greater than force down wind generated by drag on sail the board will head up and become difficult to bear away.This is usually precursor to fin generating so much lift it tries to flip board.!!! Point it happens at will be lower for lighter sailor..
There is a correct size fin for given weight of sailor/size of sail and likely speed and conditions.
Too large afin (which it is !) will make things harder. As for getting upowind on smaller fin !!! It just needs to be travelling faster to generate sufficient lift !! (ie head upwind when fully planing;which is always good technique anyhow)