Ponyboy describes his oldest brother Darry as physically resembling their father to a remarkable degree, yet being unlike him in other ways.
It seems funny to me that he should look just exactly like my father and act exactly the opposite from him.
The comparison and contrast between Darry and the boys' father is significant. Following their parents' untimely deaths in a car wreck, Darry, the oldest of the three brothers, has been thrust into a father-figure role. He has to look out for his two younger siblings, Ponyboy and Soda; he has had to assume a lot of responsibility when still quite young himself. This leads him to be rather domineering towards Ponyboy, the youngest, and Ponyboy resents this. Ponyboy feels that Darry doesn't understand him and is too harsh on him; he is not kind like their father was. Ponyboy is too young to appreciate the pressures of responsibility on Darry. It is not until much later in the book that the two brothers are brought closer together, when Ponyboy returns after running away. Darry is moved to tears:
'Oh, Pony, I thought we'd lost you ... like we did Mom and Dad ... '
Darry's emotional reaction shows just worried he has been. This has the effect of making Ponyboy realize for the first time how deeply Darry cares about him, even if he wasn't always able to show it. After this, Ponyboy finally comes to understand his brother.