Do larger eggs have larger yolks, just more eggwhite, or is there more of both inside them?

Sometimes Jumbo and extra large eggs have two yolks.

More of both! To answer your question.

Yes, more of both.

Just think ostrich vs chicken.

More of both.

And yes, more frequency of double yolks.
We got double yolks a lot, when I had half a dozen chickens. Those eggs were larger than what you find in stores. If my birds laid any that small I would have slapped them, except my Japanese bird. She laid blue eggs, or rather turquoise. They were small, but predecorated for Easter.

Fun fact, to get brighter yellow in egg yolks, feed them ground corn. They should also free range, for more satisfying flavor.

Yes. Sometimes one, sometimes another, sometimes both.