If the diameter of circle A is twice the diameter of circle B, then the radius of A is twice the radius of B.
Area is , so square the factor. The area of the larger circle is four times the area of the smaller circle.
Try it with a specific example. A: Diameter = 4, so radius = 2 B: Diameter = 2, so radius = 1.
Area A =
Area B =
Area A is four times area B.