Proportionality has nothing to do with it, that plays a role in retribution, not defence. What matters is that if the fat guy can be made to work off the damages caused throwing him out represents force greater than the minumum force needed for a successful defence. But that’s another debate.
There is no harm being done. She put the fetus in there and as long it is in there it is there by her choice. So any “harm” being done is continously being select to be done by her.
Again, this isn’t a matter of defense. It is a matter of selection, choice. It would be absurd to claim your dragging a passed out drunk from a porch is defense when it was you yourself who dragged him there in the first place. Stop demonizing the fetus! I don’t imagine even ultra-feminists do that!