My sweet little 7 lb. Belle (oddly enough her full name is Jezebel ) becomes twice her size when she plays. Katie does it sometimes too, but not as much (funny thing is she is exactly twice Belle's size - 14 lbs). I don't know how Belle does it - her tail gets HUGE! I just crack up - they look so funny. This usually happens when her and Katie wrestle. Nothing to worry about, just take some pictures and show us!