Is there a biological reason why so many animals have a pygmy or mini version of themselves? Is there a niche they fill that normal sized animals cant? Is there a pygmy gene carrier all animals have an ancestor with?
Think about humans. There are a many simple mutations that will cause human beings to be very short in adulthood (see Dwarfism - Wikipedia). If an environment exists where smaller organisms have more offspring, those genes become fixed and after several generations every member of the population will have them.
On islands large species tend to get smaller while small species tend to get larger: