The megalodon shark is known as one of the most fearsome creatures the world has ever known, immortalised in the film "The Meg".
But, after re-analysing fossil evidence, scientists now think the shark was significantly thinner than previously thought.
Previous research had suggested it could have been up to 20 metres long.
However, size estimates have varied widely as they were based on the only remaining fossils of the shark, which are teeth and vertebrae.
Scientists had assumed that the megalodon had a similar stocky body shape to its modern descendant, the great white shark.
According to a study in the journal 'Palaeontologia Electronica', a better model may be the thinner mako shark.
"Our team re-examined the fossil record, and discovered the megalodon was more slender," said Phillip Sternes, a biologist at University of California, Riverside.
"It still would have been a formidable predator at the top of the ancient marine food chain, but it would have behaved differently based on this new understanding of its body," Sternes added.
The megalodon "may not have been a powerful swimmer" compared to the great white shark, said study co-author, Kenshu Shimada, a paleobiologist at DePaul University in Chicago.
It also may have had to hunt less due to a longer digestive canal.
This could also affect the mystery of the megalodon's extinction.
It was once thought that the shark’s extinction came because there was less prey around. But its updated body image could point to another reason.
"I believe there were a combination of factors that led to the extinction, but one of them may have been the emergence of the great white shark, which was possibly more agile, making it an even better predator than the megalodon," Sternes said.
Shimada said that knowing the megalodon's true shape would require finding a more complete skeleton.
The shark went extinct around 3.6 million years ago.
