This is just wrong. The Sun is the same size (with just a little variation) through the day. It's just that the perspective of items at the horizon fools the eye sometimes. You'll find upon further examination that the Sun always appears about the same size as your thumbnail when held like the artist's method, at arm's length with thumb up.
Now they still can't explain why it's the same size in their geography, but that's another story.
By the way, I'm currently working on their Forums to apply skeptical, experiential approaches to the the problem. I'm sure I'll get banned soon, but I'm trying to keep an open mind.
Regards,
Gulliver