For height and weight, there are links to the climate that the people live in. Typically, hotter climates favor skinny people while colder climates favor more husky people. As for height, my understand is that a dry heat favors taller people as they can take advantage of sweating more than a wet heat.
I don’t think the form of Lemmy as a federated service will be able to scale.
What I expect is that, if Lemmy is successful, it will be as a platform for various Reddit alternatives, kind of like how Truth Social is Mastodon.