Recent contributions in legislative studies field have coined the term mega-seats to denote committee systems and leadership bodies. The significance of viewing the internal bodies of legislatures as mega-seats is that they are conceived as part of the democratic delegation chain. Consequently, such an approach adds a political bargaining dimension to the allocation of mega-seats. During the internal organization process of the legislature, plenary legislators become principals, who delegate power to internal bodies, mainly to enhance labor division, tackle information asymmetries, and channel party demands. This thesis examines the process of payoff distribution in legislatures, using an original dataset containing 350 parties, in 12 Western European parliamentary democracies. The analysis is carried out at the party level as well as at the legislature level. Moreover, I conduct two case studies - Portugal and the United Kingdom - to further disentangle the causal mechanisms used to explain mega-seats allocation in parliamentary democracies. The empirical analysis starts with an examination of whether the division of payoffs (i.e., mega-seats) follows a proportionality logic. The proportionality assumption is borrowed from coalition studies, which have long established that institutional payoffs are distributed in a 1:1 proportionality. Using a new index to gauge disproportionality in the allocation of legislative mega-seats, the first finding of this thesis is that mega-seats allocation in parliamentary democracies is not proportional. Subsequently, I adduce a model that explains this counterintuitive finding at the party and legislature levels. The second main finding is that a party’s degree of disproportionality is a function of its power. Parties are conceived as having a number of resources to spend on mega-seats distribution. The way they spend these resources is constrained by the existence of proportionality protection rules within an institution and incentivized by the value of the payoff. Regarding the former, I find that rules matter in protecting proportionality whilst for the latter I find that the amount of resources parties are willing to spend on a mega-seat depends on the mega-seat’s power. Finally, the third main finding is that, at the aggregate level, the overall power of the legislative branch vis-à-vis the executive branch is important in determining the degree of disproportionality. Powerful legislatures tend to be more disproportional, as executive members seek control of its internal bodies.