Why elaboration should be done with a SMALL team
A [wikipedia:RUP] project consists of 4 different phases. The elaboration phase (the second phase in the RUP project lifecycle) is normally done with a small team. One of the important reasons for this is that you want to define the basic form of your architecture. This means you think about concepts, elaborate on them, develop proof of concepts and select the ones that work best for your specific project.
If there is one thing you can not use (early) in an elaboration phase of a project, it's having too many team members. There's probably not enough work to go round. Because of that the members who do not or can not contribute to defining the architecture or developing the proof of concepts will eventually end up being assigned production (or construction) tasks.
That's where things go wrong: in an elaboration phase the architecture is designed and takes its basic form, but in construction the architecture should be stable and proven. In elaboration you define the standards and guidelines, while construction needs those things to be available. Tasks in elaboration take some time because they have to 'grow'. Because of this they are harder to plan, while tasks in construction have a hard deadline. And as one of the 'elaborators', it's pretty darn annoying when one of the 'constructors' comes up to your desk every two hours to ask if the part they are waiting on is ready. That kinda kills the whole goal of the elaboration phase...