Why are these instances so big, but not others? Like if I made a public instance and allow people to join, would it become this big?

Depends on a lot of factors. The big 3 are:

1. Reliability/accessibility. These instances all have significant capacity, good uptime ratio, connection speed, etc. All of those things necessitate expenses, of course, though not necessarily massive expenses. I believe sh.itjust.works gets by with less than 10k per year.

2. Administration. Some people want a light hand, others a heavy hand; some want minimal bias, others want to see anyone who disagrees get the boot. In all cases, an active and responsive admin team helps.

3. Communities. Power users drive social media. How active initial users and communities are drives future growth.

Oh, and all of these instances are several years old, including being extant at the Great Exodus back in '23, which significantly boosted everyone’s numbers.

Also, where do .world instances fall on this map?

Lemmy.world is in the center; other .world instances are probably similarly placed.