A Weight on their Minds

Little is know of the Westwood family from Foxton, New Zealand, but the attached photographic postcard made them famous. All right, the portrait is just of parents with their seven children. However, what is remarkable is the two children in the front, Wilfred and Ruby. At age 11, Wilfred weighs 21st 12lbs while Ruby, aged 15, is 17st 4lbs. I wonder what caused their immense weight and why just them? They must have got taller and heavier as they grew up. Does anyone know more about them>