Weight alone is not an indicator of what is healthy for an individual piggy, their heft is.
You need to put your hands around their ribs and check that there is a nice covering of fat but that you can still feel the ribs.
You do need to make sure their weight is stable (or gaining as they are still young) to ensure they are eating enough hay each week.
This guide explains -
Weight - Monitoring and Management
Feed a good hay based diet, with one cup of veg and one tablespoon of pellets per piggy and they will be getting a good diet and will be at their genetically determined size.
Long Term Balanced General And Special Needs Guinea Pig Diets
As they live outside, ensure their hutch is fully insulated and has good covers, that there is lots of hay and heatpads inside to keep them warm.