Imagine a believer initially affirming all authentic teachings of Islam in general. Later the person learns the specific commands concerning prayer, fasting, charity, or pilgrimage and comes to affirm each explicitly.
Initially: a comprehensive acceptance of what the Messenger ﷺ brought.
Subsequently: explicit assent to particular duties, their conditions, and meanings.
This is genuinely an expansion in detail, without implying that the earlier general assent was absent. “More” here measures known subjects, as when an overarching agreement is later understood in its specific provisions.