It all depends on the user activity
For example In 'Accounts' collection objects I see arrays like 'tags_usage' or 'transfer_history'
So 1 user using a lot of tags and doing a lot of transfers takes more space than 100 inactive users
I think the size of the DB will mainly depend on number of posts and comments no matter if they will be done by 1000 users or 10 very active ones

But I'm not @furion so he would need to validate this