I'm gonna argue that comparing the two are different. A person interacts with people every day. They have an idea of how large humans can get. Yes, some grow outside of the the average, but they still have an idea.As for the I didn't realise the chickens were going to grow so big...
I wonder if they say that about their children.
Many people have never seen a grown chicken in person other than a chicken that has already been butchered and cleaned in the store. Adult weights are given in catalogs, but it's not always easy to convert 8-9 pounds into, for example, a 10" wide and 16" tall bird without measuring the dang bird in person. I would be thinking of a dog or cat, which has very different dimensions because that is what I would likely be familiar with