Please weigh your piggies once weekly throughout their lives, daily when they are not well or you are suspecting a problem; always at the same time in the feeding cycle to minimise the influence of a full/empty tummy/bladder, which can make up to 30g difference.
During the first 4-6 months of their lives, guinea pigs grow very quickly. In order to cover their
small extra need of calcium and protein, you can either give them unlimited pellets or a handful of alfalfa hay daily. Most widely available pellets are alfalfa based, so if you'd rather give alfalfa hay, you need to limit the pellets to max. 40g per piggy per day. If you feed less, the quick growth phase will simply last longer until the piggies have realised their ideal body size, so it is not a matter of stuffing babies. Guinea pigs are very flexible when it comes to growth and can make up lost ground even in their second year of life if necessary.
I would recommend that you rather feed the pellets in portions than just top up the bowl, as youngsters can be scatty and soil up the bowl.
Once the weekly growth rate is slowing down or stopping, you can either stop the alfalfa hay feed or start to reduce the daily pellet intake very gradually to the half to handful (10-20g) of pellets that fully grown adults need from 12-15 months onwards.
Pellets should only make 5-10% of a good piggy diet, veg about 10-15% (i.e. ca. 50g of a veg mix that contains at least one high vitamin C veg or herb) and hay should make up to 80% of the daily food intake for good health and fitness.
https://www.theguineapigforum.co.uk...veg-and-fruit-list-with-vitamin-c-grading.42/
The healthy weight range of adult piggies can vary between 800g-1500g. Pet piggies tend to weigh about 900g-1300g on average. This is a huge variation, so it makes more sense to check your adult piggies' heft by their ribs - they should not stick out, but should still be felt when you handle them in a healthy piggy. if you can't feel the ribs, then your adult piggy is overweight.
If your adult piggies are very porky, reduce the pellet intake if it is higher than our minimum recommendation, but do it gradually or you will have a riot on your hands! Overweight piggies are at a higher risk of heart failure, not least during an operation.