OK, for the curious. I have the info so here it goes. Thought it was interesting, but that's me.
A London Zoo keeper named a baby elephant they received from Africa. The exact info is sketchy but the name given to the little calf was a derivative of African word Zulu jumba or Jumbe for chief. The name that stuck was "Jumbo", Jumbo the elephant was exibited in London and sold to PT Barnum where the elephant became an English icon. Jumbo was so popular that from then on, anything big, clumsy and large from peanuts to jet airplanes were described a "jumbo".
The Donguri line of koi have the recognizable distinction of jumbo sized koi.
As a side note, After being an animal star for many years, Jumbo met his or her maker after being tragically hit by a train.