Hi & welcome to YT!
Our Toto is small and overweight by 2.3 lbs.! Our vet is constantly scolding me and doesn't want her to weigh over 3 lbs. because of her tiny frame! We have had her on a diet since last December and she hasn't lost an ounce.
We follow the vet's directions but she is a little "couch potato" and doesn't get a lot of exercise. She was 2 yrs. in July and only eats 1/8 c. of the canned Nutro weight formula in the morning and another 1/8 c. at her evening meal. I leave her about a tablespoon of dry kibble [Yorkie 28] down and she usually don't even eat that every day! In between meals she gets tiny treats for going to her wee wee pad, etc.
If I were you, I would get my vet to check that little one over and suggest a feeding regimen based on the weight and age. Again, welcome and we are looking forward to hearing all about your baby!