how good is to divide groups

Reading about this topic in programmers exchange, I assume that maybe small groups would communicate better but some projects might be a bit bigger so you need to create a module for each team and focalize what you have. Still who will lead the group would have to create a methodology so developers could giveback with no overpressure. I think is good to be inmerse in UI and BL both so you would have a better understanding of the solution, but sometimes, when a developer is more skillful in one area than the other(design) you probably would have better results focusing people on what they feel more comfortable.