The correct statement is:
the volume of the pyramid is 1/3 the size of the volume of the prism.
This is based on the formula for the volume of a prism (which is base area multiplied by height) and the formula for the volume of a pyramid (which is 1/3 times the base area multiplied by height). When both have congruent triangular bases and the same height, the volume of the prism will be three times that of the pyramid.