Almost every subreddit is fun until it grew then it goes downhill. I agree with people not wanting this to grow like Reddit.

As why Reddit grew, Digg is one and another is the format was perfect for the time.

Although growing too large not desired for Lemmy, but theoretically if you want to grow it:

First major issues and outages need to be dealt with.

Developing and deployment best practices should be followed.

Registration must be easy and open

SEO optimization

Securing funds

Getting noticed by the media often which may require some controversy.

Mod tools and supporting brands.

As you see many of these ,at end up be bad for users.