What is your FeatureLayer.Mode? If it is OnDemand, FeatureLayer.Graphics get populated after extent change, which means your layer's FullExtent would be similar to your Map's current extent. If it is SelectionOnly, layer's FullExtent will be that of selected features only. If it is SnapShot, you can expect that after FeatureLayer.UpdateCompleted event, layer's FullExtent will be union of extent of all its graphics.