unfortunatly its because chihuahuas are such a popular "acessory" breed...
(up untill a few years ago i showed and bred long haired chihuahuas)
the current holliwood trend are these so called "teacup" "deer type" (which due to the body shape stay lighter than well bred chi of the same height/lenght)
and so youve got all these so called "breeders" breeding whats in and selling because "thats the fashion"
yes its true that standars change and shows have effect in that...
but in cases like this where its simple popularity.
my 3lb male is a 2 time akc champion, absolutly gorgoues coat, great personality, and i still have people come up to tell me that he cant be a chihuahua because his chest is to wide and his heads not the right shape...then try to tell me that their dogs smaller...
*sigh*
there so used to seeing the liked of tinkerbell and bambi and other fashionista badly bred fancy puppymill wowies that they have no idea what a well bred one looks like...
its a problem in many breeds as others have said, the akita being slimmed down, shepards being fluffed up, rotties so bulkie they look like there on steroids, apbts that look like hippos...
its all about whats in fashion...
even more unfortunatly is that its more common in toy breeds...