What precisely does dlgStretch do in Eagle PCB?
The stretch factor does not matter when there is only a single space.

For multiple stretchable spaces, the factor specifies how much of the available space is allocated to each space object.

Try something like this:

dlgHBoxLayout {
  dlgStretch(1);
  dlgPushButton("+OK")    { dlgAccept(); };
  dlgStretch(2);
  dlgPushButton("Cancel") { dlgReject(); };
  dlgStretch(3);
}