Relatively large?
Here's the NFHS numbers on high school football participation. They're going up right now, not down.
2012 was a bit of a trough, but they've gone back up and exceeded their highest points again. Youth football is growing.
http://www.nfhs.org/ParticipationStatics/ParticipationStatics.aspx/
Like Maufman said - the states that are worried about this don't produce all that many pros (or even college players).
First, the states that worry about this are generally ahead of the curve on setting cultural trends.
Secondly, last year, Florida, California, Louisiana, Texas, and Georgia produced the most top 100 recruits.
Florida - In 3 of the last 4 years -excluding 2012 where there was a drastic drop in participation - Florida has had between 40,000 and 41,000 boys play football. Thats stagnant, not "going up right now." In those same 4 years, Florida's population has grown by 4%.
California - In the last 9 years, California has had between 102,000 - 104,000 boys play football. That's stagnant, not "going up right now." In those same 9 years, California's population has grown by 6%.
Louisiana - In the last 4 years, Louisiana has had 20,000 boys play football every year. That's stagnant, not "going up right now." In those same 4 years, Lousiana's population has only risen by 1%.
Texas - In the last last 13 years, Texas has had between 157,000 - 163,000 boys play football every year. That's stagnant, not "going up right now." In those same 13 years, Texas' population has gone up nearly 20%.
Georgia - In the last 8 years, Georgia has had 32,000 boys play football every year. That's stagnant, not "going up right now." In those same 8 years, Georgia's population has risen 6%.
I have no idea how you consider this to be "growing", but I'm not seeing it. This, also, doesn't take into account the younger generation of players who are much less set in their ways then the current crop of 40-50 year old parents. The numbers are stagnating now. Let's see what happens over the next 10-15 years.