Welcome to the forum! You’ll find lots of information here.
I’ll do what I can to help.
Pellets
Please feed plain pellets with no colored, added food. The pellets you are currently using have this. Plain pellets are much healthier. I use oxbow pellets, which can be found on amazon or at pet stores. Be sure to only feed 1/8 cup of pellets per piggy, no matter what pellet you use. You would have to feed 1/4 cup daily altogether.
Cage Size
Guinea pigs need lots of room to exercise, as they cannot use running wheels. Do you know the square footage of your cage, or the dimensions?
Hay
In addition to a supply of pellets, guinea pigs need unlimited hay. I recommend buying this in bulk.
Veggies
Guinea pigs need approximately one cup of green veggies a day, each. This chart can help you out:
www.guineapigcages.com/food/Veggie-and-Fruit-Charts.pdf
Vitamin C
This is essential for guinea pigs. Some people think it is supplied through pellets, but most pellets give one guinea pig less than 7g a day. Even the good pellets don’t provide enough vitamin c for guinea pigs. I use a supplement to make sure my guinea pigs get enough. This prevents diseases.
Your guinea pig does not look overweight. Guinea pigs naturally look like that; it’s normal. Still, it’s important to keep them healthy with a correct diet and cage size. Please let us know your cage dimensions.