I'm running it in the smallest VPS of vultr with 25GB of disk.
This instance only has 3 users, with me being the only active. It says it's been up for almost a month and I've only used 3GB.

Here are the docker volumes which have the actual data of your instance, and from inside the DB the biggest table is the one called activity which the devs said it's only sometimes used to validate the data, but could be truncated if needed (there's a schedule task which only keeps up to 6 months).
Also the thing to have in mind is to properly configure the logs of whichever installation guide you follow.
After that I've seen other admins say the next biggest is the media uploaded (from bigger instances).

$ du -h --max-depth=1
640K    ./pictrs
3.2G    ./postgres
3.2G    .

lemmy=# select
  table_name,
  pg_size_pretty(pg_relation_size(quote_ident(table_name))),
  pg_relation_size(quote_ident(table_name))
from information_schema.tables
where table_schema = 'public'
order by 3 desc;
         table_name         | pg_size_pretty | pg_relation_size
----------------------------+----------------+------------------
 activity                   | 2187 MB        |       2292867072
 comment                    | 56 MB          |         58212352
 person                     | 48 MB          |         50307072
 comment_like               | 45 MB          |         47161344
 post_like                  | 22 MB          |         22781952
 comment_aggregates         | 14 MB          |         14811136
 post                       | 13 MB          |         13623296