How does the narrator know the bass on his line is very large?

Thank you for posting your question here. In The Bass, The River and Sheila Mant, the narrator didn’t want her to think he was stupid for fishing, so he tried to hide his fishing pole. When he moved it, a big fish, the biggest he ever saw, bit his lure. Instead of reeling it in, he tried to paddle the canoe in a way that would tire the fish out and keep it on the line.