Oakton's at 2,500 in 2016 and is projected to climb to 2,637 by 2020.
Westfield's at 2,565 in '16 and projected to be at 2,610 in '20.
Cville at 2,519 in '16 and projected to 2,566 in '20.
Tilly at 2,763 in '16 and projected to 2,731 in '20
Herndon at 2,307 in '16, projected to 2,485 in '20
Robo at 2,688 in '16, projected to 2,649 in '20.
Assuming these projections are accurate it looks like none of these schools has a pure numbers advantage over any other.
http://www.fcps.edu/fts/dashboard/enrollment/hsenroll15-16.html