Use the arrow on the right side of RadRibbonBar to collapse it. When click some of the tabs to show the popup.

Workaround: specify a minimum height for the popup:
        protected override void OnLoad(EventArgs e)
  {
            base.OnLoad(e);
            radRibbonBar1.Expanded = false;

            RibbonBarPopup pop = this.radRibbonBar1.RibbonBarElement.Popup;
            pop.PopupOpened += pop_PopupOpened;
        }

        private void pop_PopupOpened(object sender, EventArgs args)
        {
            RibbonBarPopup pop = sender as RibbonBarPopup;
            pop.MinimumSize = new Size(0, 150);
        }