We consider whether the manipulation of committee sizes can serve as a strategic tool of the majority party to further its influence over policy outcomes in the US House. Previous research notes the influence of the majority party’s preferences on the composition of committees but takes the size of committees as exogenous. We argue that the determination of sizes is an important first step and potential tool to shape committee composition, given vacancy constraints like the property rights norm. Using assignment and revealed legislator ideology data from the 80th to 111th Congresses, our results support this view of strategic expansion as a majority party strategy particularly for "prestige committees," which are most central to a party’s agenda. Expansion indeed results in committees that are ideologically closer to the majority caucus median.