procedure DimensionarGrid(dbg: TDBGrid);
type
  TArray = Array of Integer;
  procedure AjustarColumns(Swidth, TSize: Integer; Asize: TArray);
  var
    idx: Integer;
  begin
    if TSize = 0 then
    begin
      TSize := dbg.Columns.count;
      for idx := 0 to dbg.Columns.count - 1 do
        dbg.Columns[idx].Width := (dbg.Width - dbg.Canvas.TextWidth('AAAAAA')
          ) div TSize
    end
    else
      for idx := 0 to dbg.Columns.count - 1 do
        dbg.Columns[idx].Width := dbg.Columns[idx].Width +
          (Swidth * Asize[idx] div TSize);
  end;
 
var
  idx, Twidth, TSize, Swidth: Integer;
  AWidth: TArray;
  Asize: TArray;
  NomeColuna: String;
begin
  SetLength(AWidth, dbg.Columns.count);
  SetLength(Asize, dbg.Columns.count);
  Twidth := 0;
  TSize := 0;
  for idx := 0 to dbg.Columns.count - 1 do
  begin
    NomeColuna := dbg.Columns[idx].Title.Caption;
    dbg.Columns[idx].Width := dbg.Canvas.TextWidth
      (dbg.Columns[idx].Title.Caption + 'A');
    AWidth[idx] := dbg.Columns[idx].Width;
    Twidth := Twidth + AWidth[idx];
 
    if Assigned(dbg.Columns[idx].Field) then
      Asize[idx] := dbg.Columns[idx].Field.Size
    else
      Asize[idx] := 1;
 
    TSize := TSize + Asize[idx];
  end;
  if TDBGridOption.dgColLines in dbg.Options then
    Twidth := Twidth + dbg.Columns.count;
 
  // adiciona a largura da coluna indicada do cursor
  if TDBGridOption.dgIndicator in dbg.Options then
    Twidth := Twidth + IndicatorWidth;
 
  Swidth := dbg.ClientWidth - Twidth;
  AjustarColumns(Swidth, TSize, Asize);
end;

procedure TForm1.FormResize(Sender: TObject);
begin
 
  // É só informar o seu DBgrid
  DimensionarGrid( GridProdutos );
end;