1. Drag a RadGridView to a form and dock it to fill the form.
2. Create and set a ColumnGroupsViewDefinition to the grid
3. Add data and run the project.
4. Resize the form and you will see that grid view layout would not look properly.
Also if your resize a column group only the first column in the group is resized while all others retain their widths.

Work around:
void radGridView1_SizeChanged(object sender, EventArgs e)
{
    this.InvalidateGridLayout();
}
 
private void InvalidateGridLayout()
{
    this.bllGridView1.TableElement.ViewElement.RowLayout.InvalidateLayout();
    this.bllGridView1.TableElement.InvalidateMeasure(true);
    this.bllGridView1.TableElement.UpdateLayout();
}