Noticed the exact same thing sailing with a light freestyler on a 2008 90 Flare and 5.0 sail with RDM mast... 23 cm fin. Crazy efficiency in 15 knots, air temp 15 c, planing away, doing flakas when others on bigger stuff were gust hunting. Light sailor weight seems to help but so does light gear weight, he had carbon everything.
As do the width discussion, width and a larger fin lowers the planing threshold. Judging from the Flare's performance, board width makes a bigger differance than fin size. A narrow board requires higher speed to plane, but displaces water better than a wide board, so it "slogs" faster. A (light) guy on a 120 l freeride board might need only a 6.5 to plane in 12 knots, The same guy on a 150 GO might need a 7.5 or larger to plane in the same conditions, but the board will lift off at a slower speed ("earlier') and continue to plane at a lower velocity. A wider board requires a more powerful sail/larger sail for the same wind speed but rewards with much greater planing efficiency than the narrower board with a smaller sail.