Ideš na to dobre, ale asi takto:
najprv by som si deklaroval premennú vo vnútri triedy napríklad ako private

type
  TForm4 = class(TForm)
    WebBrowser1: TWebBrowser;
    Button1: TButton;
    procedure FormCreate(Sender: TObject);
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
      ZoomFactor: OleVariant;
  public
    { Public declarations }
  end;

potom by som jej priradil hodnotu 1 ( 1 = 100%)
//.zoom:=0.25; //25%
//.zoom:=0.5; //50%
//.zoom:=1.0; //100%
//.zoom:=2.0; //200%
//.zoom:=5.0; //500%
//.zoom:=10.0; //1000%

procedure TForm4.FormCreate(Sender: TObject);
begin
  ZoomFactor:= 1;
  WebBrowser1.Navigate('pc.poradna.net');
end;

a nakoniec ju zvyšujem

procedure TForm4.Button1Click(Sender: TObject);
begin
  ZoomFactor:=ZoomFactor+0.1;
  WebBrowser1.OleObject.Document.Body.Style.Zoom := ZoomFactor;
end;