To reproduce:
- Set the Windows theme to Windows 7 Basic
- Start a RadRibbonForm and maximize it

Workaround:

void Form1_Resize(object sender, System.EventArgs e)
{
    if (this.WindowState == FormWindowState.Maximized)
    {
        this.RibbonBar.RibbonBarElement.IconPrimitive.Padding = new Padding(5, 5, 0, 0);
    }
    else
    {
        this.RibbonBar.RibbonBarElement.IconPrimitive.Padding = Padding.Empty;
    }
}