Most of the bands that I know of, no matter what the size, are build around the interaction of two people--most rock and pop music can be constructed around the idea of two guitars, rhythm and lead, with additional instruments fleshing out the bass and rhythm and then adding soloists--

Most vocal arrangements start around the interplay of a melody and a harmony part(the second voice doesn't need to simple follow the melody a third away, and if it does, somebody better start studying music theory) with additional voices adding chordal depth and color.

The main thing is that the more instruments and voices that you use, the less there is for the other parts to do, and the more organized everything has to be to be in order to work.

The smaller the group, the more better each play needs to be, and the more opporunities there are to demonstrate musical virtuosity--the bigger the group, the greater the need for a strong leader, and ability to cooperate becomes more important than ability to play or sing--