Vyzkoušel jsem a nic. Pravděpodobně je to kvůli rozdílu Delphi 2010/Delphi 2006

Po spuštění:
992kB
Po provedení 13x CreateTabBrowser
96 376kB
Po provedení 13x CloseTab
94 464kB

procedure TMainForm.Button1Click(Sender: TObject);
begin
  CreateTabBrowser;
end;
procedure TMainForm.CloseButtonClick(Sender: TObject);
begin
  CloseTab;
end;

procedure TMainForm.CreateTabBrowser;
var
  ts: TTabSheet;
  WB: TWebbrowser;
begin
  ts := TTabSheet.Create(PageControl);
  try
    ts.PageControl := PageControl;
    ts.Parent := PageControl;
    if PageControl.ActivePageIndex > 0 then ts.PageIndex := PageControl.ActivePageIndex + 1
    else ts.PageIndex := 0;
    WB := TWebbrowser.Create(ts);
    TControl(WB).Parent := ts;
    WB.Align := alClient;
    WB.Silent := True;
    WB.Visible := True;
    PageControl.ActivePage := ts;
    WB.Navigate('google.cz');
  except
    ts.Free;
  end;
end;
procedure TmainForm.CloseTab;
begin
  if PageControl.ActivePage.Controls[0] is TWebbrowser then
  begin
    (TWebbrowser(PageControl.ActivePage.Controls[0])).Free;
    PageControl.ActivePage.Free;
  end;
end;