The primary reason brachycephalic dogs have smaller litter sizes is that their skulls are much smaller than other dog breeds. This means there is less room for the puppies' brains to develop, leading to neurological problems and congenital disabilities. Additionally, the small size of the mother's pelvis can make it difficult for her to give birth without assistance. Because of these difficulties, it is not uncommon for chihuahuas to have litters of only one or two puppies.