Q1= Are the recommended sail sizes for boards based on an average sized rider ?
A1= They are based on an "average" of various rider weights feedbacks/preferences.
So yes, in a roundabout way, they are averaged between the preferences of 60-90kg test riders, but you could consider them more aligned to "typical" use by (average) 70-80kg riders.
Q2= ...would a 7.5 be closer to the sweetspot on the F101 if it was being ridden by a 66 kg sailor rather than 80 kg ?
A2= Yes. Marginally. The extra reserve of size (planing area and volume) afforded by a lighter rider weight allows a (given sized) board to effectively carry more static and dynamic load/weight (from the bigger rig) so the max sail size (range) can be seen to increase marginally on a given board as the rider gets lighter.
With a 66kg rider, the Futura101 would be further into the sweetspot range with a 7.5m than with a heavier 80-90kg rider. Also, by using a lighter (dynamic, more than static) weight sail @ 7.5m will further optimise things..
(OverDrive is a pretty grunty sail, in powered use it can tend to load a board even more [for a given size} than some "lighter"[..less grunty] alternates )
Cheers ~ Ian