Too much Factory Capacity relative to Production results in Factory Idle Time (Idle Capacity).
In practice you will always carry a small percentage of idle time. Less than 10% is acceptable. Any more than this can be considered an inefficient use of funds. Excess idle time can be resolved through increased production (if there is sufficient actual customer demand) or selling SCU.
If you have zero idle time this may indicate you suffered Lost Sales due to insufficient Factory Capacity. You should view the Lost Sales figures on the Product Summary or Market Summary reports.