Besides the obvious size difference already discussed, the biggest causes of Lemmy feeling like a ghost town, are the splitting up of communities, both by instance and by niche, and the lack of common defaults across instances.

The first split is inherent to the design but could be solved (I think I saw a proposal for aggregating like communities from different instances that aren’t defederated), but I don’t know what the interest level is.

I feel like the second is a mental issue for everyone coming from the likes of Reddit. With the number of people here and the amount of activity you’d see, people should be much more forgiving of “off topic” posts in any given community. I’m guilty of it too, so this isn’t meant to be a slight. But the answer to someone asking what community they should post in for some niche topic shouldn’t be “create one”. It should be “post in a popular related community, they’ll be fine with it”. I could go on about the nuances, but hopefully my point is clear: hardly any communities (if any) on Lemmy are big and active enough at this point to warrant gatekeeping to keep the noise level down.

The last one would be easier if there was a solution to the first one, but having five or six communities that everyone who joins Lemmy is subscribed to by default would go a long way to making this place feel busier.