I found a data set on the National Federation of State High Schools Associations website that listed number of schools with swimming and diving programs, and the total number of participants for boys and girls. To determine average roster size, I simply divided the total number of participants by the number of schools offering programs.
The resulting graph shows a cross-over point where a number of factors caused girls’ team size to increase while boys’ team size decreased. It’s difficult to determine what factors (budget, interest, accessibility, etc.) contribute to changes in team size, but I’m guessing Title IX (1972) had a lot to do with it.