By optical illusion, do you mean that while it seems bigger, it isn't actually bigger?
Because if that is the case, I can tell you for certain it is not. I've been lucky enough to see the sunset in a number of places, such as the Sahara, Carribbean, and just recently in the Outback from Ayres Rock. All of them showed the sun to be much bigger at the horizon that in the top of the sky.