Hi
We cannot tell you from a photo unless a piggy is grossly obese.
But you can check for yourself by feeling around the ribcage at any age and size. If you can just about feel the ribs, your piggies are a healthy weight for their size. If you cannot feel any ribs at all, your piggies are overweight and if you can feel every single rib, then they are underweight. The bum shape is genetically determined and cannot be used to judging a piggy's weight.
Feeling for the 'heft') or - if you'd rather - the BMI will then help you to interpret the recommended life-long weekly weigh-in on your kitchen scales correctly and will help you stop slowly developing problems early on.
The problem with guinea pigs is that the 'average weight' range covers only half of the guinea pigs but that doesn't mean that the other half are not healthy unless the rib feel tells you so. The weight also changes naturally over a course of a lifetime.
I have had a large boar who was naturally slightly underweight at 1400g who could have easily carried another 200g without being overweight (he still lived to ca. 8 years) and I also have had several dainty sows at just around 800g who achieved the same great life span - just to show that you can only judge the weight on the scales appropriately if you know wether your piggy is a healthy for themselves or not.
Crucial for boosting health and a god natural life span is a good hay based diet with only 1 tablespoon of pellets and only a modicum of preferably green and leafy veg; pellets and veg together (plus any treats which you have to balanced out) replace the supplementary role that wild forage used to have. A good diet can add about 1-2 years to a healthy life span so diet and weight monitoring are really important.
You can find out a lot more about weight and health monitoring in these two links here:
Weight - Monitoring and Management
Long Term Balanced General And Special Needs Guinea Pig Diets
These guides are part of our New Owners helpful and very practical 'how to' information collection (which you may want to bookmark):
Getting Started - Essential Information for New Owners