One chihuahua in my pups litter had a large head, very large. They all have the extrme apple head look. But This one is almost freakishly big. Is that a sign of something bad, or just his individual look?
You might want to have your vet give him a look over. I am no expert but I have heard that they can get water on the brain. To make sure you have a normal healthy baby take him to see a vet as soon as possible. Keep us posted.
Do you know the full pedigree backgrounds on the sire and dam of this breeding? By your description, its likely that this puppy has hydrocephalus or water on the brain which causes a severly enlarged head. Unfortunately, only a vet can diagnosis this and most of these puppies do not survive or ever have a normal life. It is mostly impossible to treat and will lead to seizures and poor quality of life. Get the puppy to the vet ASAP!
Here's a link on a hydrocephalic chihuahua puppy, you can see from the pictures he has a very domed head, but they can be even more large then this;