Heft is the best gauge of whether they are a good size for themselves not weight - a piggy can weigh 1500g and be healthy and to another 900g is healthy.
Put your hand around their body and if you can feel a nice layer of fat over the ribs but still feel the ribs, then they are a good size for themselves. If you can’t feel ribs at all, then there could be an issue with their weight. But you must not do anything to change their diet without consulting a vet
Pellets will greatly contribute to weight gain and ultimately ill health if they are overfed them.
Each piggy should have a diet of unlimited hay, one cup of a variety of veg including leafy veg, and a strictly limited portion of just one tablespoon of pellets per day. A bowl filled with pellets is likely to be too much - with the pellets I use one tablespoon amounts to just 6 grams of pellets or around 15 pellets