board to small?

[Vicskim666]

Just upgraded to a M3/4 carbon poly vac from a L 5/8 ultra vac. I really like the speed and stiffness of the carbon over the ultra. But im noticing when I get into deeper water it becomes very squirrely and sinks fast. Also, having a really hard time controlling it and the slightest weight shift results in unintended movements and sudden turns. Assumed at first that board is at a higher performance level than my riding ability but then I began thinking maybe Im too big for a medium at 5'9 180lbs?

got a killer deal on it so maybe I should just lose some weight?