The size of a guinea pig does not determine its size of pellets. You may notice that an old guinea pig can have small pellets while a younger guinea pig can have more massive pellet poop. But when you notice your guinea pig producing abnormal pellets that are tiny or odd in shape than usual this, can be a cause for concern. It could be an indicator of different health problems including pain, gastrointestinal issues, and appetite loss. So, if the pellet size of your guinea pig is constant, then you have nothing to worry about even if it's small sized.