Looking at a Great Dane next to a Chihuahua is quite asight. While Great Danes often stand around 32 inches tall, a Chihuahua’s average height is under 9 inches. 

Geneticists are trying to find an explanation for this. In this age of modern domestication, why are the largest dog breeds up to 40 times bigger than the smallest? The answer is still unclear, but scientists found a growth hormone variant that traces back to ancient wolves.

So, what are the biggest dogs across the canine world? Here are the largest dogs and breeds to know. 

What is the biggest dog in the world?

The biggest dog in the world by height was Zeus, a Great Dane from Otsego, Michigan. According to the Guinness World Records, Zeus was 44 inches tall and 7 feet, 4 inches on his hind legs. 

Despite his intimidating height, Zeus’ owner Kevin Doorlag said he regularly visited schools and hospitals as a therapy dog. Zeus died at age 5 in September 2014. 

The world’s tallest living dog is coincidentally also named Zeus, a Great Dane from Bedford Texas. At just 3 years old, Zeus is about 41 inches tall and stands at 7 feet, 4 inches on his hind legs. 

Zeus’ owner, Brittany Davis, was gifted the pup when he was 8 weeks old. Today, he enjoys walks around the neighborhood and the local farmer’s market and sleeping by the window. He eats 12 cups of “Gentle Giants” dog food per day, Davis told Guinness. 

"The comment that we hear most often is 'Wow, that’s a horse!' 'Can I ride him?' or 'Does he have a saddle?'" she said. "The answer to all those questions is 'no.'"

According to Guinness World Records, the longest and heaviest dog ever recorded was Aicama Zorba of La-Susa, an Old English Mastiff owned by London resident Chris Eraclides. In 1987, Zorba weighed 343 pounds and measured 8 feet, 3 inches from nose to tail. 

Know your pup:Dog life expectancies based on breed, size and more

What is the biggest dog breed?

The English Mastiff is the breed with the largest dogs. Male Mastiffs typically measure about 30 inches tall, and female Mastiffs will normally clock in around 27.5 inches high. The average weight for an English Mastiff is 160-230 pounds for a male and 120-170 pounds for a female dog. They typically live between 6 and 10 years, the American Kennel Club says. 

Here are some of the largest dog breeds by weight and height:

• English Mastiff: 27.5-30 inches and up, 120-230 pounds
• Pyrenean Mastiff: 25-31 inches, 120-240 pounds
• Spanish Mastiff: 28-35 inches and up, 140-200 pounds
• Irish Wolfhound: 30-32 inches and up, 105-120 pounds
• Neopolitan Mastiff: 24-31 inches, 110-150 pounds
• Saint Bernard: 26-30 inches, 120-180 pounds
• Great Dane: 28-32 inches, 110-175 pounds
• Roman Miortic Shepherd Dog: 22.5-29 inches, 100-130 pounds
• Newfoundland: 26-28 inches, 100-150 pounds
• Leonberger: 25.5-31.5 inches, 90-170 pounds
• Anatolian Shepherd: 27-29 inches, 80-150 pounds
• Scottish Deerhound: 28-30 inches, 75-110 pounds
• Dogue de Bordeaux: 23-26 inches, 99-110 pounds and up

What is the smallest dog in the world? 

The world’s smallest dog is Milly, a 1-pound Chihuahua born in 2011. The Guinness World Record-holding pup stands just under 4 inches tall. Chihuahuas are the smallest dog breed in the world and typically weigh no more than 6 pounds.

Other end of the spectrum:Here are the world's smallest dog breeds

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