Please take deep breath and calm down! Your girl is growing at an ideal rate for her age, which is the important bit. You are doing everything right. Your girl won't reach adulthood until she is 15 months old. With your good and measured care, she still has over a year to make the best of what genetic markers she has been born with, individually and in her own good time.
I have adopted litter triplets 3 years ago at 8 weeks of age. The weight difference between the heaviest and the smallest was 100g. This has got even bigger as time went on (in fact, it has doubled), but - and this is the important bit - all the sows have made it within the acceptable range of adult weights, which is very wide, and they are all a good and perfectly healthy weight/size ratio in themselves. It doesn't matter what another guinea pig weighs; as long as your guinea pig is healthy and happy in itself, there is no reason to worry!
Human children are not all the same weight and size at any age, either, are they? Nor are they all born the same weight and will be the same weight as adults (in fact, adults can weigh a multiple of each other, and it is not necessarily the biggest ones that are the healthiest). It is the same with guinea pigs!
I have even had a pair of litter sisters where the difference in weight at the peak of their lives was a whopping 500g. But it was the smaller energetic sister, Nia, that was actually the leader of a large group, and not big Nerys. Both have lived to a good age; one to nearly 6 years old and one to 8 years of age. There you are!
Weight and size aren't everything, you know. Health and a good diet that allows your piggies to get the best out of what genetic dispositions they have been born with are much more important for long term health and longevity. It is better to aim for a slow, sustained growth than trying to fill your piggy with empty calories, so it becomes obese and lives a shorter life.
I have several girls that just about creep into the 800g limit, but they can still live to a good old age and be perfectly healthy. My sisters Hedydd and Heulwen are in fact celebrating their 6th anniversary this month, and while they are struggling to stay this side of 800g in their old age, they are perfectly fine in themselves and are still going strong. I am much more worried about their not related age mate Hafren (the third of my piggies celebrating her 6th birthday this month), who has been on the chubby side and is looking rather frail and old now.