Rate me autistic or disagree, but making your pet obese is abuse in my eyes. You control their food and many animals will eat themselves to death if you allow them. Chihuahua are supposed to be 8 pounds at most with their ideal weight being 6 pounds. Twinkie looks to be about 15 pounds, possibly more. I can't really tell if she's naturally large for a chihuahua because using Amber's gigantic mass for scale doesn't help. She's like the dog equivalent of Amber's weight. Remember the shitshow of Rarity's spaying or this current nails debacle, image what will happen if Twinkie gets diabetes.
People love their pets and they show their love by feeding them treats. Having a fat pet doesn't mean you hate your pet. I don't think Amber maliciously abuses and neglects her pets. She's just self-centered, stupid and immature. She prioritizes wants over needs and responsibility.
Edit: After I wrote my autismo out I remembered the Gracie incident. She's fully capable of being malicious towards pets as long as it's someone else's pet.