Hmm, I'm not sure I've ever heard too much about this but I can speak from a bit of recent experience. I have a pup that is also 15 weeks, I got him at 10 weeks and his legs grew much longer in a short period of time but it is sort of like the rest of puppyhood - some things grow faster than the others - which explains the big puppy ears and big puppy paws that don't seem to match the rest of the bulldog body.
In any case, I would say my bullies stopped getting taller around 7-8 months and then it took another year to fill out completely with those typical bullie features. Bulldogs tend to stay "puppies" a little bit longer than other dogs - in mentality and in stature - in my opinion.
I don't know this is helpful but are there specific reason you are concerned, at this point, about vertical height? If your concern is growing too fast, make sure you are not feeding the puppy foods that are too high in protein.