Imagine an aerial view of the pyramid so that the vertex (point) of the pyramid appears to be in the centre of the base at the intersection of the diagonals. If the pyramid has no height the 4 faces of the pyramid are the four triangles created by the intersecting diagonals of the base. So in this case the total area of the faces is the same as the base area. Now imagine pulling up the point of the intersection of the diagonals to make the vertex of the pyramid. This will stretch the triangular faces so that their combined area is greater than the flat base. So the sum of the areas of the triangular sides is greater than the area of the base.