library cache values in statspack report

I notice in my statspack report, under SGA Breakdown section, "shared pool library cache" shows begin value = 139M and end value = 3MB over a period of 3 days. It does not look normal to me. Could someone shed some light on possible causes of this?

Thanks,

Is there any way to set a minimum size for library cache? I could not find a parameter that can do that.

Thanks