Weight, size and shape will vary a lot from piggie to piggie (just like people really).
As long as she has an appropriate diet, no underlying medical issues, and plenty of room to run around in, then she would be unlikely to be fat (just like people really).
I have 4 adult guinea pigs and Lucy has steadily weighed between 1000 - 1050 grams for the past 12 months.
Oreo on the other hand (our most active piggy) weighs around 1350 grams.
Ruby and Eddi fall somewhere in the middle.
All are perfectly healthy.