Everything, even the disk space consumed by the installation (although that is trivial and static). The data in the database is quite compressed or efficient though and the only thing you should worry about is attachments (of any type).
Assuming you have an issue that is really simple and has, for example, a 40 character summary, 200 character description, an 80 character comment and a worklog, the raw data in there is 40 + 80 + 200 + 200 for the text, plus a couple of bytes for comment author and a few more for the comment date, and similarly a few more for the date author and amount of time done in the worklog. Say 350 bytes overall.
But it will take up less space than that, but also more.
More because there's data related to how it is being held, and data you have not entered directly but still needs to created and stored. A field has field and screen configuration at the very least, and if you're talking custom fields, context, options, searcher, name etc.
But because all of that is stored in a database, the storage is mostly optimised. Try looking at the physical files that the database server stores the Jira data in. They look like junk to the human eye because they're built for optimised access first, and minimal space second.
What I'm saying here is that the rules for working out "how many bits something will take up on a disk" are fantastically complex and not worth the effort.
Yes, the more issues/pages/users/other stuff in the database you have, then the more storage it consumes. But it is trivial compared to the attachments and repositories. Hundreds of thousands of issues remains a small amount of disk space, unless you decide to add 1Mb attachments to each one
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