
I have the same with one of my larger Chihuahua girls. She had difficulty whelping her first litter a couple of years ago, and I had her spayed after pups left home.
Mine get fed twice a day, and, because I work, my sitter does one feed, and I do the other. I weigh food out religiously, but suspected that he had stopped weighing, and was just guessing. I forbid him to feed her at all. I feed her myself, once a day, and even took her away with me, so I knew she could only eat what I gave her. She has had no perceivable weight loss, and I feel really horrid about the small amount of food I'm giving her.
I even gave her extra exercise while away, and had her on 45gm wet food, and 6 gm of dry. If you can find an answer, do let me know.