Yes, but that's not the point.
The point is to keep the servers relatively small so that the non profits can keep breaking even on nothing but donations, even with an influx of new users entering the fediverse.- bigger instances should ideally only be trying to grow when their donations are more than covering the costs. (That being said I wouldn't be surprised if the bigger instances started having problems what with their seeming ability to continually accept new users without closing once)
In the grand scheme of things the bigger instances having 20K users isn't a whole lot, and can be done using smaller servers - the thing with social media is that usually only about 20-25% of people are actually "active" - the rest are lurking or dead accounts and maybe occasionally commenting.
The smaller instances (like the one Im on) have anywhere from 1-1000 users and are highly unlikely to fall outside the range of the low cost of a little bit higher than a hobbyist side project, and what with the tendency for smaller instances to have more % of their members also be donators probably never have to fear running out of money
It really only becomes an exponentially expensive problem when you reach twitter and Reddit levels of users on the same instance - as you end up needing more and more expensive custom load balancing and caching solutions in order to keep up with the demand - basically it's more sustainable for a few thousand people to support the costs of a 1000 instances with 100,000 total members than it is for a company to try to make a profit off of a single monolithic structure supporting the same number of users.
The fediverse splits this load across servers, even segregates it. There are areas of the fediverse that I will never see due to a lack of direct connection through the nature of how your feed works, this helps as not everything is needlessly routed through a single point.
Also Wikipedia faces the same issues and still manages to get through - sure they put up banners asking for donations when margins are getting in to what they consider the "danger zone" but usually the danger zone is more than what's required.
- Basically they always over budget so they never go under.