Thank you all. There is probably a relation with wetted area as Belskorpio says, although I would think that the larger surface has more grip/drag whatever the pressure (static or dynamic). The analogy with a blade suggest that the narrow board will resist more with its leeward rail, until it reaches a certain speed. Once fully planing, a few more knots may explain that you get similar lift from a smaller fin, while the sideways force of the sail does not increase in proportion.
Ken, I also found the selector pretty useful so far (but I use mostly Select fins).