[anonymously]

Nutrisca Salmon and Chickpea kibble, check the reviews on chewy.com. It is small kibble, ideal for small dogs. No potato, no grains, no chicken….

PS: Don’t free feed. Offer measured amounts twice a day or whatever your vet recommends. Some senior small breeds do better on 3 small meals a day, add a splash of water to meals or presoak the kibble. Maybe add a topper.
Has the dog had a senior workup, labs and an exam to rule out medical issues for the weight gain. I would start there.
Is she getting enough exercise, brisk walks….