My sense is that the bigger the userbase, the more it attracts junk, spam, abuse, etc. So, the rules get tightened to combat it. Also, my impression is that the moderation tools are not great, so crude/heavy-handed methods are sometimes all that is available.
I think you found the corollary already: smaller subreddits have less rules and/or less strict enforcement.
I'm not sure a better solution, given the situation. Though I agree it can be discouraging for well-meaning occasional contributors.
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For example, I wanted to post a funny Risitas youtube vid I made (you know the mexican comedian with that laugh...), and couldn't find a single usable "funny" subreddit. Some banned youtube content completely. Some banned "video memes", some banned X and some banned Y... all of them had slightly different guidelines and you immediately got an insta-splurge from a bot-mod if you tried posting. Some required you to prefix every post subject with some code word. I had to give up eventually and post it on some super-small subreddit instead that accepted anything.