Just to keep it fun.......
Big fins don't cause a board to go upwind, the sailor steering the board causes the board to go upwind. However, a big fin will go upwind easier/better than a small fin. The excessive lift from a large or small fin given enough speed can cause the board to become unstable and ultimately "turtle" or flip the board because the fin is trying to reach the surface.
Most sailors when overpowered with or without too much fin intentionally head upwind to reduce their speed trying to regain control, but it's not the fin causing the board to go upwind.
I tend to sail with larger than normal fins and the board always goes where I steer it, regardless of the fin. As I said before, I use a 42cm fin in my iS 111 in 20-25 knots of wind with a 6.6 sail and the board always goes where I point it regardless of the lift from the fin. I have done some tail-walking, but always in the direction I choose.
Formula 160; iSonic 111; HiFly Move 105; Tiga 263; '85 Mistral Superlight.
Maui Sails TR 11.0; 9.2; 8.4; 7.6; 6.6; Maui Sails Switch 6.0; 5.2; Maui Sails Global 4.5; 4.0.