A simple matter of size :)

Gulliver's Travels, Jonathan Swift's satiric masterpiece, has always fascinated this writer regarding the relative sizes of the Lilliputians vs the Brobdingnagians as the former are about 6 inches tall while the latter tops out at about 72 feet.

The King of Brobdingnag and Gulliver by James Gillray (1803), (satirising Napoleon Bonaparte and George III). Metropolitan Museum of Art

Seems an astute scientist did some rather interesting research regarding the intrepid Lemuel Gulliver and the remarkable nations he visited  To whit ...

"The sizes of the Lilliputians and Brobdingnagians are close to those of rats and large dinosaurs, respectively."

Gulliver is described as being less than 6 feet tall in the book. Per Kuroki, a 2000 survey of heights across England found that the average height for men in the 18th century was around 5 feet 6 inches, so this seems about right. Apparently Swift "adopted the base of 12 in imagining these fictitious peoples," Kuroki wrote. Per Swift's text, the Lilliputions were 1/12 the size of Gulliver, while the Brobdingnagians were 12 times his size. That would make the Lilliputions just under half a foot, with the Brobdingnagians towering above everyone at just over 67 feet. Both fictional species are described as having bodies shaped similarly to humans.

Swift assumed weight would be proportional to the cube of the height, and used that to estimate the relative energy requirements of the three species. But Kuroki cites Quetelet's rule (1869), stating that weight is proportional to the square of the height. It was kind of the body-mass-index (BMI) of the 19th century, and Kuroki came up with an average BMI of 23 for his analysis. That would put the average Lillipution's weight at 1 pound, Gulliver's at 148 pounds, and the Brobdingnagians at 21,351 pounds. 

Interesting to a fault as is the book.  :)