To reproduce: please refer to the attached sample project and gif file.

Workaround:  this.radRibbonBar1.RibbonBarElement.ApplicationButtonElement.DropDownOpened+=ApplicationButtonElement_DropDownOpened;

 private void ApplicationButtonElement_DropDownOpened(object sender, EventArgs e)
        {
            foreach (RadItem item in this.radRibbonBar1.RibbonBarElement.ApplicationButtonElement.DropDownMenu.Items)
            {
                RadMenuItem menuItem = item as RadMenuItem;
                if (menuItem!=null)
                {
                    menuItem.MinSize = new Size(item.Size.Width, 24);
                }
                
            }
        }