The reason PG ratings seem consistent with where kids end up in college has a lot to do with exactly that. Grades are MUCH better for committed kids (especially commitments to P5's and other strong baseball programs) who share the same or perhaps worse numbers than uncommitted players.
This is especially true with rankings. They will adjust quite a bit after the MLB draft as well. This makes the projections look very good historically.
But yes, size is definitely a factor! Upper 90's exit velocities for players who are 6'-2" and 205+ don't impress me; they do impress PG scouts though. If you are 6'-2" and weigh 205+ you SHOULD have an upper 90's exit velocity or you are not working very hard. Kids in the 90's who are average height and weight or less (6' and under and 140-180 lbs) impress me. You'll notice the vast majority of players who commit early are already or near average college playing size. No size projections needed.