I'm not sure why it would grow but why you have it detached from the featuremanager?
Does it only grow when docked in this position next to the task pane? I just tried to reproduce the width growing and it did not grow after using multiple sketch entities, creating different feature types, etc. I also can't find any logged SPRs (bugs) relating to this type of issue in our system.
Are there certain commands or actions that make it grow wider that you can list? If not, perhaps keep an eye on it to see if you can determine what action makes it grow wider and then post back with this information.