Well generally the fin range indicated is linked quite closely to the sail range, ie. smallest fin recommended will probably be about right for the smallest sail recommended.
I'm guessing that a heavier sailor sailing a 6.5m well powered up, for instance, will probably need a bigger fin than a lighter sailing a 6.5m well powered up as for the lighter sailer the leverage of his weight on the rig will be much less significant and therefore so will the sideways forces on the board and he won't need as big a fin.
I guess this is sort of like me taking a 29cm fin with a 6.0m when most sailors would need something bigger.
JP Super-X 106l, North F8 Ram: 5.8m,