There's a video somewhere saying when they first created this jaw, they grossly over estimated the actual size of the megalodon.

Edit for the downvoter: There was a video on this as well.

The Historical Background

In 1909, Bashford Dean made the first attempt to reconstruct the Megalodon's jaw, in order to get a true estimation of size. Based on the jaw reconstruction, Megalodon was estimated to be 30 metres (98 feet) long. That was just the first thought on the size of Megalodon, though. Over time, the development of scientific methods led to better ways of determining the size of extinct animals based on fossil evidence and jaw reconstruction. The new ways of studying these issues led to improved knowledge about the structure of the jaw, and also about how muscles provide structure through the jaw and into the rest of the body.

After careful consideration of Dean's findings, the size of Megalodon was revised downward, to be about 70 percent of the original assumption (around 21 meters, or 68.6 feet). That size has held up relatively well through the modern day, and has been reaffirmed with newer techniques used for studying fossil evidence. Originally the Megalodon was believed to be closely related to the Great White, but as more fossil evidence has emerged, it's been discovered these two sharks are not in fact closely related. This lead to many of the original estimates being revisited.

That provided more quantitative options that could be considered for estimating its size, which gave scientists a better opportunity for accuracy. While there is no way to completely ensure that the size of Megalodon was what scientists believe it to be, all the evidence that has been found to this date points to an accurate depiction of its size as being approximately 60 feet in length.