Oh, that's what you're referring to. I was thinking of that too, although I didn't make the connection that that's a hint that God of War Jötnar can shapeshift. Good point. It could just be a Loki thing, though, since if I recall correctly, he was particularly adept at shapeshifting even among his kind.
On the topic of Norse mythology's Giants, it's cool to think how many abilities their kind has. They've got shapeshifting, prophecy and foresight, and illusion-making. That's only from the stories I've read too, so for all I know they've done other things too. The gods obviously have more incredible powers, but there's something about it coming from a completely different, non-divine race.
Yeah the shape shifting angle is the one that makes most sense to me, and to be honest this is nothing new for God of War- remember Cronos and Rhea from God of War 2? Here's a reminder.
Being of the same race and being brother-sister and also husband-life no less, its clear that these beings can shapeshift or at the very least change their sizes at will.