The dimensions of the 18-inch cake and the double-layer cake are different, so the sizes are also different.

An 18-inch cake refers to a circular diameter of 1 inch equal to 0.75 inches, which is 2.54 centimeters when converted into centimeters. An 18-inch cake is 18*2.54 centimeters = 45.72 centimeters).

There are many sizes of double-layer cakes, such as 12+10 inches. The total circular diameter is 1 inch equal to 0.75 inches, which is 2.54 centimeters when converted into centimeters. A double-layer cake is (12+10 )*2.54 cm = 63.18 cm).

It can be seen that in terms of circular diameter, a two-layer cake is much larger than an 18-inch cake.