Owl's big eyes are to allow them to see clearly in low light conditions. Their eyes are adapted to gather more light, giving them an advantage when it's dark. Also, big eyes aid in spotting prey from a distance.
The reason could be that big eyes for the owl in stories symbolize its ability to see clearly in the dark, making it seem mysterious and powerful. Also, it makes the owl visually appealing to children.
Undefeated Owl was a short drama. You could see some information about Undefeated Owl, such as the plot, the main actors, and the way to watch it. However, these search results did not provide any specific information about watching Undefeated Owl in one go. Thus, I didn't know how to finish reading Undefeated Wild Owl in one go.
Well, if Chatnoir opened his eyes during Dark Owl fanfiction, it might lead to a surprise twist. Maybe he'd see something crucial that others missed or have a new realization about his abilities.