Twin fins are bigger than your tri-fins to compensate for the lack of a thruster fin.  The twin fin setup is extremely maneuverable but during a powerful bottom turn, the board is likely to slide out and loose control.  For more stability or on those bigger days add a Vektor fin to hold the tail down and give you more of a positive traction in your turn.  The small fin depth and template still allows the board to pivot like the original design and feel of the twin fin.