Genetics: Some guinea pigs are simply smaller than others. This could simply be due to the genetics of your pet.
Diet: Guinea pigs need a diet that is high in hay, as well as a variety of fresh fruits and vegetables. Make sure that you are feeding your pet a healthy and balanced diet.
Exercise: Guinea pigs need to exercise regularly to stay healthy. Make sure that you are giving your pet plenty of opportunity to run and play.
Stress: Stress can also cause a guinea pig to stop growing. Make sure that your guinea pig is in a comfortable environment, with a quiet and safe place to sleep.
If you are concerned about your guinea pig's growth, it is best to consult with a veterinarian.