The external surface area of 1 g of colloidal clay is at least . . . . . . . . times that of 1 g of coarse sand

The external surface area of 1 g of colloidal clay is at least 1000 times that of 1 g of coarse sand. This is because of the small size of the clay particles whereas the size of the sand is quite bigger than clay.
Particles less than 0.001 mm in size possess colloidal properties and are known as soil colloids and these are the finer size fractions of the clay soil.