The narrative case is used to describe things that one did not witness themselves. In English we usually discern this by using "apparently" or "evidently" (as in, I see the evidence, but not the process).

If the speaker of this sentence is Dino's grandmother , who has not seen Dino for months, she can only comment on the evidence (-m|ş) because she was absent for the process (-|yor).

But in Turkish you must be explicit in how you came to that Dino grew. İf you witnessed him growing yourself, you could say "Dino çok büyüktün."

However, if you only noticed a difference in height after not seeing him for a few yearsbor if you heard of his increased size from someone else that's indirect knowledge. You would therefore say "Dino çok büyümüşsün."

"Anlaşılan" is added to tell the learner that the knowledge is indirect so it can be translated using the narrative.

O kadar büyümüşsün = You have grown that much

"Dino o kadar büyümüşsün ki..." "Dino ne kadar büyümüşsün" "Dino ne kadar da büyümüşsün" these forms are true, their meanings are same. But I can't explain why, i am sorry.