This article examines the relationship between polity size and political trust in local government for which two schools of thought offer opposing expectations. The theoretical disagreement has received considerable attention and this article contributes with a dynamic perspective of how changes in polity size affect changes in citizens' political trust on the input-side of local government. The case examined is the recent municipal mergers in Denmark which were implemented on 1 January 2007. The article reports an analysis of a panel based on a repeated survey of Danish citizens using an untreated control group design with pre-test and post-test. A quasi-experimental difference-in-difference identification strategy is used to obtain the effect of municipal size on local political trust. The results show that changes in municipal size negatively affect local political trust, which is consistent with the expectation from the political economy theory that political trust tends to decrease with the size of the polity.