This paper reports the results of an empirical investigation into the role of project scale, as reflected in consortium size, on the impacts obtained by partners participating in publicly-funded collaborative R&D projects. I argue in this study that scale may affect performance indirectly rather than directly. Specifically, I model the influence of scale as being mediated by a set of intervening variables that may be said to “transmit” both positive and negative effects through (i) complementarity of resources, (ii) learning, and (iii) transaction costs in project implementation. Moreover, I hypothesize that these indirect effects are conditional on certain moderators that include resources committed, project management mechanisms, and project uncertainty and scope. The results offered in this study largely confirm the proposition of conditionally-mediated effects of scale on performance. They indicate that a number of conditional indirect effects are indeed significant, and surprisingly, that these effects are mostly negative.