  Be X-ray binaries (BeXRBs) consist of rapidly rotating Be stars with neutron
star companions accreting from the circumstellar emission disk. We compare the
observed population of BeXRBs in the Small Magellanic Cloud with simulated
populations of BeXRB-like systems produced with the COMPAS population synthesis
code. We focus on the apparently higher minimal mass of Be stars in BeXRBs than
in the Be population at large. Assuming that BeXRBs experienced only
dynamically stable mass transfer, their mass distribution suggests that at
least 30% of the mass donated by the progenitor of the neutron star is
typically accreted by the B-star companion. We expect these results to affect
predictions for the population of double compact object mergers. A convolution
of the simulated BeXRB population with the star formation history of the Small
Magellanic Cloud shows that the excess of BeXRBs is most likely explained by
this galaxy's burst of star formation around 20--40 Myr ago.
