Hi Reynald,
When you fetch an aggregate on a screen, the platform knows which fields of the aggregate are used and only fetches those used attributes from the DB. The query in this case is optimized.

However, if you isolate the aggregate in a server action and use that server action in a data action for instance, the platform doesn't know which fields are used, and all attributes are fetched.
This doesn't apply to CRUDs, only Gets (I think that was your confusion).
Hope this helps,
Pedro