I havent got the answer but I think its sort of other way around. Smaller boards just dont seem to work with big fins.When I`ve experimented its not been railing or problems getting planing thats been issue its been during gybes. Board just dropped straight off plane with too large a fin mid gybe;tail sinks and you always end up flare gybing. I think this scenario is something to do with larger higher drag fin needs width to maintain planing when off the power ???
Theres more to it than board needing width to act as extra leverage on big fin.(Which is obviously part of it) ????????