Where do pure breed large dogs come from?

Just a few days ago I suddenly realized one thing. Most Taiwanese prefer smaller dogs like Corgi, Puddles, Pugs, the traditional Taiwanese black dog etc etc, and when I happen to pass by a pet store I can usually see those puppies on sale.
But then, how come there are so many pure breed dogs of larger breeds around the island? At work I have a Belgian Shepherd of human-like size that was found abandoned. Not far from my office there are a mastiff and something that I don’t know what it is, looks like a Golden Ret. but bear-sized.

Are they all “imported” by plane/boat?