They look great!
I sowed Russian Giant last year, and in the awful wet that we had I was surprised that they even flowered, but they did - they weren't amazing, but to be honest if they'd been as big as yours we wouldn't have had room for them! The flowers were around breakfast plate size.
This year, as the last snows melted I saw some little self-sown sunflower seedlings popping up - I didn't have the heart to weed them out and sow fresh, so I let them be and kept an eye on them. They're flowering now - the biggest is about 7ft tall and the smallest about 2ft6ins. The biggest flower is the size of a small teaplate and the smallest is ........ well, quite small indeed.
So my guess is that seeds from yours will produce smaller flowers next year than you get this year - if you're saving seeds, save those from the biggest flower and obviously save the biggest fattest seeds.