[[coso]]

debes usar el sender como ttreeview,

Código Delphi [-]
procedure TForm1.TreeViewAmpliar(Sender: TObject);
begin    
(sender as TTreeview).Top    := 100;    
(sender as TTreeview).Left   := 100;    
(sender as TTreeview).Height := 500;    
(sender as TTreeview).Width  := 500;
end;

antes, lo que estabas cambiando era las propiedades de 'self', o sea, de la form1