Aren't idol groups best with 5 members or fewer?

If the roles are divided into vocal, rap, dance, visual, and the "fan magnet," it seems perfect, but with 7 or more members, I can't tell who is who. They all look the same to me.
In the case of very large groups, is it because agencies find it difficult to keep trainees stored away indefinitely, so they just bundle a few of them together and release them all at once?