Chihuahua puppies share certain qualities with all other dogs as puppies. They're formed a certain way to make birthing them easier.
All puppies, no matter the breed, are born with round heads, thick, short muzzles, relatively short torsos, and very short legs in relation to their bodies. They need to be as streamlined as possible to exit the birth canal.
It's only as they grow that their features become more apparent. So seeing a chihuahua puppy at 6 weeks is not really going to give you a very accurate indication of what they'll look like as adults.