The difference is in utility of smaller instances as a themed community where you might find companionable strangers, vs mastodon.social as a polyglot grand central station. In a big instance, the instance timeline isn’t of much value except as “oh look, thousands of people saying stuff in many languages”.
Which is also nice, obviously.
Consider this a friendly, local pub. Make yourself at home, bring your friends, have a good time! Meet new people, have a laugh, enjoy the ambience, and the Oxford commas.