How leatherback turtles grow huge on a diet of jelly

Imagine eating nothing but jelly all your life. It’s hardly the richest source of food, and you might expect an animal with such a menu to be small and feeble.  You’d be wrong. The leatherback turtle eats jellyfish and little else, but it grows up to 640 kilograms in weight, and can migrate over thousands of kilometres. How can such a powerful giant subsist on such ethereal meals?