EMC. What Serena was saying is correct. People who breed "larger" or "smaller" dogs than what the Chihuahua is supposed to be according to Standard (there are actual ranges that need to be followed if you ARE breeding for the betterment of the breed, and if you aren't, well then you are indeed an irresponsible owner).
And your comment about not breeding Chihuahuas because they don't conform to the standard..I disagree strongly with this remark. A responsible and reputable breeder will only breed dogs for the betterment of the breed, combining careful and selective breeding, like Serena mentioned. If a dog is too large or too tiny and they are being bred regardless, they are simply backyard breeding the dogs and have absolutely no knowledge of the breed or whats best for it, to begin with. If a breeder is truely a responsible one, they will do thorough health checks on the dogs, including hips, thyroid, gawd only knows how many other exams are involved.....and only when the dog is proven to be completely healthy and stable, only then are these dogs bred, in turn decreasing the chance of having a dog with genetic or health problems...of course it is a dog, and dogs get sick just like people..so we can't expect some sort of a robot. But, a good breeder will and does ensure that the quality of the breed will increase, not decrease...or they just don't breed at all.