The breed standard for French bulldogs calls for a maximum weight of 28 pounds, and no official designation of a teacup variety has been determined. "Teacup" is simply a marketing label implying small size.
French bulldogs were originally bred as a miniature version of the English bulldog. They normally weigh 19 to 28 pounds. While healthy but smaller than average puppies sometimes appear in normal litters, unscrupulous breeders may sell runts, premature puppies or malnourished puppies as teacups. Another strategy for producing smaller than average puppies is to breed only from parents that are smaller than average. Unless a breeder is careful to select for health and proper conformation as well as size, this can lead to health issues.