well we believe that adam was extremely tall!
Volume 9, Book 87, Number 171:
Narrated Samura bin Jundub:
So we proceeded till we reached a garden of deep green dense vegetation, having all sorts of spring colors. In the midst of the garden there was a very tall man and I could hardly see his head because of his great height, and around him there were children in such a large number as I have never seen.
he could hardly see adam (AS)s head! so imagine how tall he was. So the idea of giants mustav been adopted from the earlier generations of men.
hope that answers your question!