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