It's not uncommon for guinea pigs to have smaller or "runt" individuals in their litters, as seen in studies on 'small for date' guinea pigs. Similar to other animals, guinea pigs may experience size variations within a single litter, with some individuals being smaller than their siblings. However, being smaller doesn't necessarily mean these guinea pigs are unhealthy or have developmental issues. As a responsible pet owner, monitor your guinea pig's growth and provide proper care to ensure its overall well-being.