How did half giants come to be?

Like what do you mean half giants? Half elves sure, dwarves being diluted ofc, more exotic partners? Humans x Minotaurs? x Beastkin? Whatever. But giants?? How did their blood get diluted/mixed? Actual giants are stated to be about mountain size. The logistics simply seem insurmountable.

Some theories I thought of are:

GDI gave someone sex skill.

Some ritual that makes the human more tall and giant like.

Similarly, some horrific blood magic shenanigans.

The other half species I could envision the progenitors of, being normalish couples. The creation of half giants I can only imagine to be horrific or completely ludicrous.