or it could be because they spend all their time looking up, seeing that they are so small
ROFLOL!!!! There you have it. The answer!
No, seriously...I have never heard this. (doesn't mean it's not true)These dogs are
ANCIENT, it's ancestors dating back to the Toltec civilization. This is also the oldest breed on the American continent and the smallest breed in the world. I think my Chihuahuas are sturdy little chunks. But they get a 2 - 4 mile hike just about every day. They don't have fragile, long necks at all. I don't see their heads as being that disproportionately large either, a little and....ears yes. LOL. (All the better to hear you with, my dear) But I guess it's their forheads that are rather large and they often need a C-section when giving birth.
Now, my Doberman....there's a long neck. And they do sometimes have spinal problems. (CVI) But they're also quite a recent development. Unless dreadfully, poorly bred, the Chihuahua is quite an evolved and well adapted dog. They aren't as whimpy as one might think. And yes, they're meant for a companion and they DO make great ones!