"The most merciful thing in the world, I think, is the inability of the human mind to correlate all its contents. We live on a placid island of ignorance in the midst of black seas of infinity, and it was not meant that we should voyage far. The sciences, each straining in its own direction, have hitherto harmed us little; but some day the piecing together of dissociated knowledge will open up such terrifying vistas of reality, and of our frightful position therein, that we shall either go mad from the revelation or flee from the light into the peace and safety of a new dark age." - Lovecraft, about how this reproduction works.
I'm not a FGO player, so I'm out of the loop. Do Da Vinci and her alter act as mother and daughter in the game? They seem to often get tagged as mother and daughter here.
The small one is a kind of clone created by the big one with memories backed up to a certain point that takes over when the big one gets an arm through the chest. Essentially, same person.