Hi
Are you sure of the age or weights? These are more like the average weights of 5 weeks old guinea pigs than 5 months old ones.
More important than the absolute weight is that they are a good weight/size ratio and that you feed a healthy hay/grass based diet with a modicum of mostly green veg and very little pellets (encourages eating more high fibre hay, which the teeth and digestive system are laid out for and which is crucial for a healthier and longer life). Don't be tempted to stuff your piggies with empty calories; they are still young enough with about a year to go until they reach adulthood to realise their own genetically determined optimal size and health weight on a normal but good diet that concentrates on long term quality.
Here is our weighing guide, which answers all questions on how to feel for the heft (i.e. judge whether your piggies are healthy in themselves, whatever the weight or size), what to look for in your weekly-weigh in and when you need to see a vet/step in with support feeding until and during treatment; how weight changes during a life time and all questions about over- and underweight, enrichment, healthy and unhealthy foods and treats:
Weight - Monitoring and Management
Here is our comprehensive diet guide, which looks at diet as a whole and at each food group in practical detail:
Long Term Balanced General And Special Needs Guinea Pig Diets
A good diet can in my own experience add 1-2 years to a normal healthy life span and take your piggies from the lower end to the upper half or even beyond.
You may also be interested in learning about prey animal instincts, how to work around them (like during pick up) and how you can use whispering techniques to reassure your piggies but also make them comply better in their own language instead of waiting until they have figured out humans.
Understanding Prey Animal Instincts, Guinea Pig Whispering and Cuddling Tips
How To Pick Up And Weigh Your Guinea Pigs Safely
Arrival in a home from the perspective of pet shop guinea pigs
Here is the link to our comprehensive practical and helpful new owners information resource, which you may want to bookmark, browse, read and re-read at need:
Getting Started - New Owners' Most Helpful Guides
All the links with the practical little tips and details that we cannot repeat in every post in this thread are also included in our how-to guides collection.