Split twelve participants into two groups and two discussions can run at the same time. More people get a chance to speak. Two topics can progress instead of one.
Spoken discussion does work well in small groups. So why not keep dividing the workshop until every breakout is small enough for turn-taking to work naturally?
Dividing into breakout groups undoubtedly allows more conversations with more airtime per participant. But it does so by sacrificing the shared reasoning the group needs for deeper understanding.
Breakouts split the perspectives. Each discussion needs enough of the relevant knowledge and perspectives to understand why an issue has priority, why judgments diverge, or what sits behind a problem. The smaller the groups become, the more likely it is that someone holding an important part of the picture is sitting in another breakout.
Anyone taking part in one breakout discussion can contribute only to that discussion, even when their knowledge and perspective would matter elsewhere. The more the workshop divides participants and topics, the more likely it is that expertise and insights are missing elsewhere.
Moving between breakouts does not solve the problem. First, participants cannot know when and where their perspective will be needed. Then, a participant arriving in another breakout has missed the reasoning that brought that breakout to where it is. The group may need to recap; the newly arrived participant may repeat points already made. Switching groups comes at a high price. Yet it does not remove the bottleneck; it only shifts it: the participant is now tied up in another discussion and is likely to be needed elsewhere.
Facilitation cannot be split. You too can only be in one breakout group at a time. In the others, you cannot see when a discussion stalls, drifts, gets bogged down, or needs help moving forward.
And someone usually fills the gap. It may be the most senior person in the group. It may be the most confident or simply the most vocal. Before long, the breakout has its own informal facilitator—and the hierarchy or dominance patterns the workshop was meant to manage creep back in.
Reporting back loses the reasoning. Each breakout group needs time to prepare what it will report. That time comes out of its discussion time. The reports are then delivered one after another. That takes time too. Two breakout groups require two reports; three require three. The more discussions run in parallel, the more workshop time is subsequently needed to bring their results back together.
Breakout groups can still make sense despite these drawbacks when they function as independent working groups tasked with producing specific outputs. The question then is whether those groups need to work simultaneously in the workshop. If working groups meet at different times, participants can contribute to more than one group where needed.
The problem arises when several breakout groups are meant to develop deeper understanding of multiple topics that the whole group then builds on in subsequent tasks. Then the information loss between the breakout groups and the plenary becomes the core problem. Conclusions can be summarized. But much of the reasoning, trade-offs, objections, and connections that led to them inevitably stays behind. The plenary cannot develop the deeper understanding it needs for the subsequent work of the workshop in this way.
The more detailed the report-back, the more time is lost again. The more concise it is, the less shared understanding develops. If the plenary then has to discuss the reports to follow the underlying reasoning, the idea of saving time by splitting the group defeats itself completely.