  We derive upper and lower bounds for the vertex-isoperimetric number of the
incidence graphs of unitals and determine its order of magnitude. In the case
when a unital contains sufficiently large arcs, these bounds agree and give
rise to the precise value of this parameter. In particular, we obtain the exact
value of the vertex-isoperimetric number of the incidence graphs of classical
unitals and a certain subfamily of BM-unitals. In the case when the maximum
size of arcs in the unital is relatively small, we obtain an upper bound for
this parameter in terms of the vertex-isoperimetric number of the incidence
graph. We also determine the exact value of the vertex-isoperimetric number of
the non-incidence graph of any unital.
