the "strongest" is a genetic mutation which causes them to have "double muscles". there have been a few around the world. The mutation happens to the production of Myostatin, which is the protein that blocks grouth of skeletal muscle
that is NOT the biggest dog, its a photoshop. the largest dog was is an English Mastiff named Zorba that weighs 343 in the UK. Its argued that its the owners fault.
the fattest dog is the stupid owners fault that should be shot (owners).