Narrated AbuBarzah: AbdusSalam ibn AbuHazim AbuTalut said: I saw AbuBarzah who came to visit Ubaydullah ibn Ziyad. Then a man named Muslim who was there in the company mentioned it to me.
When Ubaydullah saw him, he said: This Muhammad of yours is a dwarf and fat. The old man (i.e. AbuBarzah) understood it. So he said: I did not think that I should remain among people who would make me feel ashamed of the company of Muhammad (peace be upon him).
Thereupon Ubaydullah said: The company of Muhammad (peace be upon him) is a honour for you, not a disgrace. He added: I called for you to ask about the reservoir. Did you hear the Apostle of Allah (peace be upon him) mentioning anything about it? AbuBarzah said: Yes, not once, twice, thrice, four times or five times. If anyone believes it, may Allah not supply him with water from it. He then went away angrily.
By saying he "heard and understood it", it is saying that the description was accurate. His only thought was that he didn't want to be around those who made him feel ashamed of the company of Muhammad. If the description wasn't accurate, you'd expect him to object or think "these people have no idea what they're talking about" or whatever, right?