Wonderful pick for this exercise! Good observation on line weight, it's definitely key to giving the object dimensionality and volume. Something else about Kley that I think adds a lot of the personality and style to his sketches are his searching lines, like you can see around the elephant's right shoulder or its feet. It gives the impression that he is gliding his pen across the page in exploration before tightening the final drawing, which provides great insight into his process. Practicing this can be beneficial in improving flow, line confidence, and imaginative drawing. I would also highly recommend Marshall Vandruff's series exploring Kley if his art is of interest to you: https://youtu.be/NJYnHNCmHFY