Standard studies of multiple unit auctions generally overlook the strategic role of bidders' quantity decisions. Using a simple equilibrium model of bidding I analyze bidders' incentives to choose quantities in discriminatory and competitive auctions. The main result is that bidders have a stronger strategic incentive to bid for fewer units in competitive auctions. Since under competition a bidder pays the lowest accepted price for each unit she gets, she may benefit from dropping her quantity to let lower-valuing bidders enter the set of winners. This prediction is consistent with empirical observations from foreign currency and spectrum auctions.