Fin size in relation with board/sail

[Farlo]

Hi all, after reading ten pages of "keep going upwind" one question reminds in my mind. If you look for fin size advice, for instance in Select's selector (http://www.select-hydrofoils.com/en/...d-slalom-race/) you find that the ideal fin for a 7.2 m² sail is 35 for a 60cm, and 45 for a 80cm wide slalom board, while it would be 25cm on a 90L wave board!!

I understand it has to do with OFO and the lift required to extract the wider board from water, however the difference is huge. Simple logic would say that you need a certain amount of lift to resist the sideways force of a given sail whichever the board width. One reason could be that you go much faster with a narrow board so you get the same lift from a smaller fin (which in turn makes you go faster) but I still wonder. Roger, Ken, Coachg, others, do you have an explanation?