Dwarf Rat??? Help

I have a major question seeing my little guy has had us stumped for the last 6 months. Kieshu was born to a feeder rat mother and a fancy rat father, both of average size.. We didn't even know he was a boy until he reached almost 5 months of age... Yea Kieshu was previously known as Julliet for his small size and kinda dainty build 8O ... Kieshu is less than half the size of any female we have let alone the males he is in the cage with. Also recently One of the his sisters( Missy.. 3 times his size) Had babies and one is taking after what Kieshu looked like when he was younger. Can a dwarf rat be born to normal parents?