Communities naturally change as they grow. I'm sure I'm not the only one who can remember being able to read every single Tweet as the scrolled by on the Twitter homepage, before it became necessary to pick and choose who to follow. Or who watched popular BBS boards fission as the load got too high.
The challenge is to have healthy growth. Thoughtful moderation goes a long way (unless you're looking for a free-for-all environment, which some people are). Welcoming and engaging people who you want to encourage to set the tone helps too. I suspect the fact that in the Fediverse you can "vote with your feet" and switch servers without losing access to previous discussions may also help to limit the spread of toxicity.