If Marinette is 14-15, then that means she’s basically done growing. She’s at the end of her puberty. Sure, she can grow out and develop more as she gets older, but she’s not going to get any taller. She is already one of the tiniest people in her class.
If Adrien is 14-15, then that means his puberty has just started. There is already a height difference between Marinette and Adrien. He has to bend over as Chat Noir so that Ladybug, who is standing on her toes, can whisper in his ear. We know how tall Gabriel is.
When Adrien is done going through puberty, there’s going to be an extreme height difference between Marinette and Adrien.