The last time I bought dog food for my pup I got Mature Dog Food since she is 8 years old, but I've noticed her gaining weight. Since she is a smaller dog, weight gain is not good for her health. Is the mature food making her chubby?
It's good that you're aware of the undesired weight gain. Puppy food would be more likely to cause weight gain than adult dog food - if it was the same line (brand&nutritional profile). However, some brands can add more weight than others, especially if it contains simple carbohydrates such as potatoes in the ingredients.
Check that the following is not in the dog food/treats given: wheat, corn, soy,... high fat items like peanut butter, cheese, cream cheese, ice cream, milk, butter, dark poultry meat, poultry skin, ham, sausage, bacon, deli meat, salami, human jerky, fatty meat scraps, hot dogs and meat by products, cooked bones, pizza, nuts, chips, sugar, syrup.
If the amount of food and treats your pup hasn't changed, that's worth discussing at your next vet exam. The vet may offer a recommendation based on your specific dog's needs.
The new change in dog food probably is making her gain weight. She is old enough to be on a senior diet but you may want to consider a weight management diet instead. Also daily exercise is very important in helping your dog to lose weight. Make sure you are not over feeding your dog. If you message me you dogs weight I can do the math and tell you how many calories she should eat per day. Then you would divide that by the amount of calories per cup it says on the bag. If your dog is still hungry after you feed him on the weight management diet green beans are a great way to full up his tummy. They are high in fiber and will help his hunger. Also it is very important that you do not feed her table scraps. Let me know if you need any help! It will take time & patience!