Note on the size of a stable matching

Consider a one-to-one two-sided matching market with workers on one side and single-position firms on the other, and suppose that the largest individually rational matching contains $n$ pairs.

We show that the number of workers employed and positions filled in every stable matching is bounded from below by $\lceil\frac{n}{2}\rceil$ and we characterise the class of preferences that attain the bound.

We then identify the minimum number of equilibrium pairings that must be ``sacrificed'' when maximising the employment rate is the objective; if each stable matching is of size $\ceiling{\frac{n}{2}}$, then no such pairs appear when all vacancies are filled.