Despite recent advances, women are still underrepresented in legislative assemblies across established democracies. We develop and test a novel theoretical argument about the role played by polity size in explaining these variations. All else equal, as polity size increases, the the number of potential candidates per seat, increases. In systems where male party gate keepers dominate nominations for public office, this should lead to lower women’s representation. This effect, we also argue, is only partially offset by the increase in the number of seats that typically follows an expansion of the electorate. To avoid the selection bias inherent in estimating the net impact of increases in polity size on women’s representation, we exploit two Swedish merger reforms, one in the early 1950s and one in the early 1970s, which substantively increased municipality size in ways that were largely exogenous to the doings of local political actors. The effect of increasing polity size relative to assembly size on women’s representation is negative across both reforms. Polity size emerges as an understudied source of unequal representation.