The five-year-old Blue Great Dane who lives in Gedling, Nottingham in England with his owners Vinnie, 46, Dixie Monte-Irvine, 39, and their three children, eats 30kg of food a month.
They were shocked to find on a trip to the vets this week that he weighed over 95 kg (15stones 6lb), three stone heavier than the world's tallest dog, the metro.Co.Uk reported.
"He is a big dog and we were told from when he was a lot smaller that he was always quite heavy for his age," Vinnie said.
The family's other dog, 15-year-old Patterdale Terrier Fifi, has become a surrogate mum to Balthazar who the family got when he was just six-weeks-old.
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Despite his size Vinnie said Balthazar is a gentle giant who likes to play with their children and is best friends with the family's three pet cats.
"We have to buy him special dog beds online and he regularly eats them. He lives in the kitchen which thankfully is a big kitchen but it can make a cooking a challenge at times, she said.
The Guinness World Records do not keep a record for the heaviest dog due to animal welfare reasons.
While not Britain's tallest dog - that crown goes to an 8ft Great Dane in Swansea - experts believe he is the country's heaviest.
Last summer a three-year-old Great Dane called Major was in the running to be crowned the world's tallest dog.