Workaround:
public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();

        this.AdjustPaddings(0);
        this.AdjustPaddings(3);
    }

    private void AdjustPaddings(int index)
    {
        RibbonTab tab = ((RibbonTab)this.richTextEditorRibbonBar1.CommandTabs[0]);
        RadRibbonBarGroup group = (RadRibbonBarGroup)tab.Items[index];
        foreach (RadElement el in group.Items)
        {
            el.Padding = new Padding(3, 1, 3, 1);
        }
    }
}