When do political communities support territorial amalgamation in referendums? Existing research has identified heterogeneity between communities as a key hurdle for integration reforms. In this paper, I demonstrate that asymmetries between communities do not necessarily impede political integration. An analysis of 1500 municipal merger referendums held in Swiss municipalities since the new millennium shows that voters’ support for integration is lower when potential merger partners are poorer or larger than their own community. However, the effects of wealth and size differences are interdependent. They reinforce each other’s effect on merger support if they point in the same direction, for example, in a relatively rich and relatively small community, but they can also cancel each other out, for example, in a relatively small, but relatively poor community. These results have important implications for understanding under which conditions integration reforms are politically feasible.