This is controlled and overridden by Windows. Under some versions of Windows, you may be successful by creating a message handler to respond to the WM_GETMINMAXINFO message: Example:

{Place this code in the private section of the Form declaration}
procedure WMGetMinMaxInfo(var Message: TWMGetMinMaxInfo);
  message WM_GETMINMAXINFO;
{Place this code in the Form implementation section}
procedure TForm .WMGetMinMaxInfo(var Message: TWMGetMinMaxInfo);
begin
  with message do begin
    MinMaxInfo^.ptMaxSize.x := 20;
    MinMaxInfo^.ptMaxSize.y := 20;
    MinMaxInfo^.ptMinTrackSize.x := 20;
    MinMaxInfo^.ptMinTrackSize.y := 20;
  end;
end;