This paper discusses variation in the size of nominalised clauses (NCs) in Khalkha Mongolian and ties it to genitive case assignment and possessor agreement. Khalkha NCs display a double dissociation of possessor agreement and genitive case assignment on subjects, as i) genitive case is not obligatory in NCs containing possessor agreement morphology, and ii) genitive case is possible in NCs without possessor agreement morphology. Further, genitive subjects are impossible in adjunct NCs containing the same possessor agreement morphology as argument NCs. To account for this double dissociation and argument NC-adjunct NC asymmetry, this paper proposes that genitive case assignment should be dissociated from possessor agreement: the former depends on the presence of D, while the latter is controlled by Poss. This paper argues that Khalkha argument NCs project a full DP structure which contains PossP; thus they exhibit possessor agreement and genitive case can be assigned to subjects within them. On the other hand, adjunct NCs project smaller PossPs which lack a DP layer; thus, while they exhibit possessor agreement, they lack a DP layer to assign genitive case to their subjects. Further evidence is provided from binding that supports a difference in size between argument vs adjunct NCs.