Discussions taking place within this forum are intended for the purpose of assisting you in discussing options with your vet. Any other use of advice given here is done so at your risk, is solely your responsibility and not that of this forum or its owner. Before posting it is your responsibility you abide by this Statement
How old are they, how long have you had them and how much are they weighing at the moment. Are you offering unlimited grass hay as their main source of food? Are you weighing your piggies once weekly, as we strongly recommend?
Please answer the above questions because your post does not contain enough information; it is too scant to even guess.
Please also take the time to carefully read our weight guide, so you can check out for yourself whether your girls are a good weight for their current size ('heft' or BMI). You can only do that by feeling around the ribcage. what exactly to look out for, how weight changes over the course of a life and the wide range of perfectly healthy and normal (but outside the narrow 50% average band) weights can be found in the weight guide. You should find the guide very helpful and informative. Here is the link: Weight - Monitoring and Management
Overall it is much more important that your piggies are healthy and a good weight for their individual size than that they comply with a far too narrow concept of 'average'. But we cannot judge what is going on without more crucial background information from you.
Body size is genetically determined and not something you can influence much. We measure development in weight gain and not growth for that reason. My smallest adults have been about half the weight and size of my largest - but both were perfectly healthy and lived a normal life span.