I'd venture to say that your big fish have come in, spawned, & left. This group is the smaller/newer year class, and they may be spawning or just feeding.
Generally speaking, Crappie run in year classes ... of approximately the same size, as far as schooling up together. When they're caught from a specific spot, and sizes are mixed, I believe it's just a matter of both schools occupying the same area ... attracted by the same factors.
... cp