Overfeeding your guinea pig is very easy to do. They are constant eaters and it's easy to give too much food. A guinea pig's diet should be equivalent to 80% hay, 15% fresh vegetables and herbs, and 5% of commercial pellets. Overfeeding can harm your guinea pig's digestive system and cause obesity. To avoid overfeeding your guinea pig, all you have to do is serve them the appropriate sized meals and cut down on non-essential sweet snacks.