A possible answer could be culled from the Sichos Mussar on Parshas Chayei Sarah (שיחות מוסר, מאמר יא (פרשת חיי שרה), 'מידת ההסתגלות'). He has a shmuess entitled 'מידת ההסתגלות' - 'The Trait of Adaptability'.
He asks how was it possible, that Sarah Imeinu who was on a greater level of nevuah (prophecy) than Avraham died immediately upon hearing the news of the Akeida and yet Avraham was fine.
HaRav Chaim Shmulevitz זצ"ל observes that when being informed of his task, אברהם seeks constant affirmation as tries to understand what is being asked of him. אברהם is instructed in the following manner; "קח נא את בנך את יחידך אשר אהבת את יצחק" – “Take now your son, your only son, the one whom you love – Yitzchok.” At each step, אברהם has to qualify whom Hashem is referring to:
“Take now your son” – אברהם has to ask which one, יצחק or ישמעאל?
“Your only son” – אברהם again needs further clarity, they are both only sons?
“The one whom you love” – Like any good father, he loves both his children.
“Yitzchok” – Finally, אברהם is given a definitive answer.
The carefully drawn out manner of the question ensured that it cushioned the blow and made it more manageable. Whereas for Sarah it was said immediately as a matter of fact which came as such a shock that it was this that killed Sarah.
So possibly, in the first instance, it needed more words / stages to ensure that it would not shock Avraham to such an extent that it would kill him, whereas in the second instance as the event was over, it needed less descriptors to summarise the action.
