Earlier quoted context omitted.
The problem with generalizing about Reddit is not its size, it's that Reddit is made up of a bunch of separate forums with separate mods and rules and users and so forth. It's just a place where discussion boards live. Reddit : forums :: Blogspot : blogs.
I used to make more or less this same argument, by analogy to Usenet, phpBB forums, etc. My take now is that this falls apart in that the pathologies of reddit are down to a shared platform (the voting system, moderation tools, etc.), userbase, and site culture. While it's true that userbase and culture are differentiated between subreddits, they aren't really isolated enough, and maybe more importantly, things like…
I don't buy this, not a whit. I think /r/the_donald and /r/woodworking are as different as any two phpbb messageboards on the pre-reddit internet ever were. What is the same between them (the voting, mod tools, etc) are not opinionated enough to affect the culture, they are the bare minimum necessary to make the site work.
I don't think Reddit has changed as much as you think it has. It had its share of racists and trolls back in the halcyon days you remember, and it dealt with them (or didn't) pretty much the same way back then as it does now. If anything it is doing better on that score. But I do agree that something has changed, and I have a theory on what that is, which I will now share with you (and the one or two people who ever read HN comment threads more than a day old):
I think that what's changed is the, shall we say, meta-conversation about Reddit in the larger media. For the past 2-3 years or so, the question, "Is Reddit racist/sexist/etc?" has been a newsworthy one. Like any newsworthy topic, it has generated commentary: pro- and con- essays, nuanced opinions and dumb ones, insightful well-researched articles and misleading clickbait journalism, blog posts, FB comments, etc, etc. It spawned numerous sub-controversies (Did Ellen Pao make things better or worse? Was banning /r/whatever censorship or not?), each of which was a new opportunity for all of the bloggers and journalists and randos to weigh in again and argue some more.
Point is, if you follow this meta-conversation, by reading articles about Gamergate and Ellen Pao and so forth, you are exposed to much more of the worst Reddit has to offer than you would've been simply by using Reddit as a discussion forum to talk about fly-fishing or programming or whatever it is you went there to chat about. From the sounds of it, you did follow this stuff, at least casually. And it seems plausible to me that that changed your opinion more than any real change in the underlying "character" of Reddit (to the extent that such a thing even exists).