When Clifford was first born, he was the smallest puppy among his siblings, thus making him the runt of the litter. He was chosen by Emily Elizabeth Howard as her birthday gift.
Due to Emily's immense love and care, Clifford grew to a colossal size. Forcing Emily Elizabeth and her family to leave their small apartment in the city and move to Birdwell Island.
Ever since he was a puppy, Clifford has been show to be very kind, helpful, and friendly. He enjoys playing with his dog friends and helping Emily Elizabeth whenever he can. He also has a strong belief in doing what he thinks is right.
Clifford can sometimes be prone to getting in trouble due to his size or even being tempted to do what his friends suggest.
Due to his unusual size, Clifford does have a large doghouse and food dish that account for his size.
Clifford has only gained weight twice in the series canon: once as a puppy and once when he was full-grown.
In the "Clifford's Puppy Days" episode: "The Cookie Crumbles", Clifford briefly gained weight after eating a bunch of cookie scraps that fell onto the floor.
More famously, Clifford gained the most weight in the main series episode: "Tummy Trouble" after eating a large amount of Bowser Bites dog treats and getting a big stomachache.