Digby: The Biggest Dog in the World (1973)

Before his adventure begins, Digby is given to Bill by a friendly vet, but as his grandfather doesn't like to have an English sheepdog in his house, he is forced to give him away. He gives him to a man who works in a defense lab that experiments with animals and plants for space travel. A liquid growth formula called Project X is made there, and the animal trainer steals some of it to boost his own tomato crop. Digby accidentally drinks some and, as a result, is growing all the time. Eventually, he's the size of an ocean liner. As weird animals are attractions, he is stolen by two well-dressed hoods and sold to a circus, from which he escapes, hunted by the Army and Air Force.