Why isn't my pugs face wrinkly??

I bought a pug at a pet shop. He came with AKK papers. But he just doesn't look as wrinkly as other pugs. And he is skinny. His vet says he is perfectly healthy, and that most pugs are over weight.... But I kind of think that maybe he is not pure bred. How can I tell?.. Why isnt he wrinkly?.. Can pet stores sell dogs that are not pure bred with papers?