The IHSAA allows schools to bump up a class for I believe two years at a time.
Based on enrollments, Yorktown would be a "big school" in a two class system and Mater Dei would be a small school. The cutoff for two classes would be about 700 students(meaning about 150 teams have 700 students or less and 150 have 700+). Yorktown has 801 students and Mater Dei has 529 per this year's release.
There are more potential wrestlers walking the halls than you want to admit. Remember you don't have to be tall, big, athletic, etc to be a wrestler. Wrestlers are all shapes and sizes, which makes it very enticing to smaller kids especially.
When a school starts having success and other kids see it, they want to be a part of it. It is drastically harder to recruit kids for a struggling program, than a good one. When kids start qualifying for state or getting a lot closer to going to state you'll see more buzz about the program within the school. That will lead to more participants at the high school level and even at the youth and middle school levels.