The Chihuahua is a lot of dog in a little body. Feeding your tiny dog correctly and consistently makes a huge difference in his health.
The American Kennel Club breed standard for the Chihuahua states unequivocally that the dog can't weigh more than 6 pounds. While that's the standard for the dog show ring, when you're dealing with one of the world's smallest breeds, an extra pound or two is a big deal. If your dog weighs more than 8 pounds, especially if he looks chubby, talk to your vet about putting him on a diet.
Smaller is not better when it comes to Chis. Very tiny dogs, weighing under 3 pounds, often don't live as long as larger Chihuahuas. One meal a day isn't enough for the Chi's metabolism. Your vet can recommend a feeding schedule and the best food for bringing your tiny Chi up to a healthy weight.