In most cases, the answer to this question is because you are using Federation Relays. Federation Relays exponentially increase your server resource usage, and if you want to keep your costs low, it's best not to use them.
Another reason could be that a user or several local users follow many remote accounts or accounts with a lot of activity. Similarly, if a local user is followed by many remote users, this can also have an impact. Obviously, if your local users publish a lot of content, that will also increase your storage usage.
You can reduce your server usage by adjusting your server Content Retention. Read Content Retention for details.
Please email firstname.lastname@example.org if you have any questions.