that all depends on the instance. small does not mean it will go away. for example my instance is topical. by design, even if it gets “popular” it has some in-built upper limits and if the mass grows beyond them it means I can likely get help paying for the next steps up.
just because an instance is big does not mean its necessarily safe or stable, first its imporant to note that large instances have scaling issues as the deployment for the system is not ready to scale that way, instead they need to deploy to every bigger servers in an inefficient manner or spend a ton of time rolling bespoke deploys. these big servers are just a few volunteers. some big instances are managed by 1-4 technical people, the same numbers a small instance has.
Also it costs money to run large scaled systems, you can run an instance for you and some friends for nearly free if you find a deal and only a few bucks a month if you dont.
So big instance/ small instance does not mean much with stability, they both have thier issues. Something to note, smaller instances are MUCH easier to run.