I'm using r/cats as an example of an interest that would attract a specific demographic. Thus the membership would not be representative of the general population, no matter how large it grows.
I am not attempting to correlate r/cats membership to dating subreddit membership.
I am willing to bet that r/cats skews heavily female. I say that as a female who owns cats. I am willing to bet k-pop skews heavily young and female. I am willing to bet Tarot and Astrology do, too. I am willing to bet that rates of depression and anxiety run higher in r/frugal and r/povertyfinance than in the general population.
The people attracted to certain subreddits will be skewed based on the topic, and will not be a normal distribution, no matter how many members there are. Exceptions might be general interest default subreddits like r/pics and r/askreddit.