The volume of a triangular pyramid (or tetrahedron) is one-third the volume of a triangular prism with congruent bases and the same height. Therefore, the relationship can be expressed as:
\[ \text{Volume of the pyramid} = \frac{1}{3} \times \text{Volume of the prism} \]
In fraction form, you can say:
\[ \text{The volume of the pyramid is } \frac{1}{3} \text{ the size of the volume of the prism.} \]