Two heads might be better than one, but a recent study finds that this doesn't necessarily scale. Researchers from Oxford, University of Florida and Columbia Business School have found that being able to communicate with other people across a networked structure can indeed facilitate the formation of new ideas by the group as a whole, the more dense this network is, the less innovative each individual person is within it. Why? Essentially, the issue is group think. The researchers found that, when exposed to other people's ideas, you're more likely to think not of entirely new ideas, but ones that are somehow connected to the others, due to something called social convergence. Whether consciously or unconsciously, the things already being discussed by the group tend to be perceived as the limits of what the group can discuss, which leads to redundancies in thinking. Even people who do have ideas outside these perceived limits become hesitant to bring them up because either they feel it's too far afield, or because other powerful personalities crowd them out. This effect can be mitigated, however, by limiting the pool of already existing ideas people can see, according to an article in the Harvest Business Review by the people who led the study. The partial connection allows for the diversity of ideas needed to facilitate innovation but, at the same time, prevents thinking from being overwhelmed by the sheer mass of the group as a whole, according to the article. 

With this in mind, perhaps collaboration would be best served, then, if it was done through several smaller teams, rather than one big team encompassing everyone working on the same project.