du -skh index store redolog db
store (the actual message) is deduped. You get one 24MB copy (with base64 overhead) there.
redolog saves a copy for crash recovery... then either throws it away upon log switch on OSS edition, or archives it for NE point-in-time restores.
index is not deduped,
afaik. Each of those 300 new users would initialize their index. Even a mostly empty index has a minimum size. Send a second 20MB attachment to the same 300 users, and I'd expect a smaller increase, closer to 50MB (MIME overhead+redolog).
I wouldn't expect db/data to grow appreciably, but the first message to a mailbox(group) might trigger something.