To reproduce:
- Add CommandColumn to the grid
- Observe the size of the button in the newrow row

Workaround:
- Add some padding in the cell formating event

 void radGridView1_CellFormatting(object sender, Telerik.WinControls.UI.CellFormattingEventArgs e)
        {
            GridViewNewRowInfo row = e.Row as GridViewNewRowInfo;
           
            GridCommandCellElement cell = e.CellElement as GridCommandCellElement;

            if (row != null && cell != null)
            {                
                cell.CommandButton.SetDefaultValueOverride(RadItem.MarginProperty, new System.Windows.Forms.Padding(0,0,1,0));
            }
        }