Tyrannosaurus rex achieved its massive size due to an enormous growth spurt during its adolescent years.
T.rex may have gained up to 2.1kg per day over four years
US researchers say the carnivorous dinosaur's enlargement resulted in a monster weight gain of up to 2.07kg (4.6lb) per day over four years.
The team analysed bones from 20 specimens of tyrannosaur (the group of dinosaurs to which T. rex belongs) to reconstruct their growth histories.
Details of the work have been published in the science journal Nature.
Gregory M Erickson of Florida State University and his colleagues used growth lines on fossilised bones to estimate the ages of the dinosaurs they came from.
They then measured the circumference of the bones and used both sets of data to generate growth curves for the creatures.
The results showed that T.rex had a maximum weight gain of 2.07kg between the ages of about 14 and 18, allowing it to reach maturity in two decades. After this, the dinosaur's growth settled down.
The accelerated spurt could result in a total gain of 3,000kg.
Tyrannosaurus rex weighed around 5,000kg - sometimes more - and lived for up to 28 years.
The carnivore outstripped its relatives in size
Until now, two theories had been proposed to explain the gigantic size of T. rex. Either it grew faster than its tyrannosaur relatives or it grew for a longer period of time.
In addition to Tyrannosaurus rex, the researchers also studied bones from three other species of dinosaur: Albertosaurus sarcophagus, Gorgosaurus libratus, and Daspletosaurus torosus.
The scientists found that these dinosaurs had maximum growth spurts ranging from 0.31kg to 0.48kg per day.
Like their relative the T.rex, they could keep this spurt up for four years.
But because they were gaining significantly less weight per day, their growth spurts failed to keep up with those of T. rex, which went on to outgrow all its tyrannosaur relatives.