Yes, once you in the group, you only need to publish a single event that all members of the group can then decrypt and view. 

The largest event is actually the welcome/invite event because it has the whole tree/group state included in it. It doesn't start to become an issue until the group is ~150 (based on the max size of nostr events) but there are a few ways I've thought about to get around that so it's not that concerning.