From what I understand, dragon souls and powers are the main reason why Ruo has higher stats than he should. Even all that talk at the beginning of the game about him going beyond mortal limits because of elixirs, ointments, etc. was a cover-up. (Of course, I say this because of various speculations and conjectures, based on character lines and information from the author. It's not really definitive, so I could be wrong)
As for his physique, from what has been written, he "grows" faster, he is lucky and blessed, he understands and is naturally attracted to the sword and his techniques are more powerful if they are related to swords. He was born with his physique.
Edit: In fact, we can even assume that Ruo only possesses the dragon souls thanks to being a Dao Child, since TY said that he didn't know the method for fusing the dragon souls, he obtained such a method thanks to a dream, "an enlightenment from the Heavens"