no, too big is not a good thing

That's the case by definition... if a fin is defined as too big then it is by definition not a good thing.

Logic old sport.

technically speaking bigger fins operate at lower pressure to produce the same amount of lift as a smaller fin and/or produce more lift. Slower speeds tend to favour bigger fins due to the fact that wetted surface area drag predominates at higher speeds, but there are other variables. If a smaller fin is being asked to produce more lift than it is able to it will lose efficiency via high induced drag and cause more drag than a bigger fin.