Extensive research has been conducted over the past years to improve our understanding of sustainability conditions for large-scale collaborative projects, especially from an economic and governance perspective. However, the influence of recruitment and retention of participants in these projects has received comparatively less attention from researchers. Nevertheless, these concerns are significant for practitioners, especially regarding the apparently decreasing ability of the main open online projects to attract and retain new contributors. A possible explanation for this decrease is that those projects have simply reached a mature state of development. (Marwell & Oliver, 1993), (Oliver, Marwell & Teixeira, 1985) note that at the initial stage in collective projects participants are few and efforts costly; in the diffusion phase, the number of participants grows, as their efforts are rewarding; and in the mature phase, some inefficiency may appear as the number of contributors is more than required for the work. However, a less positive interpretation is that the projects have become inefficient in their practices, affecting both output and ability to recruit and retain participants. In this paper we use original data from 36 Wikipedias in different languages to compare their efficiency. We chose Wikipedia because the projects are at different states of development, but are quite comparable on the other aspects, providing a test of the impact of development on efficiency. Results confirm that most of the largest Wikipedias seem to be characterized by a reduced return to scale. As a result, we can draw interesting conclusions that can be useful for practitioners, facilitators and managers of collaborative projects in order to identify key factors potentially influencing the adequate development of their communities over the mid to long term.