2013 Bikes

"With rim brakes forks are loaded in bending at the fork crown that similarly supports road shock, while a disc brake places an equal bending torque at the tip of the fork and only on one blade. The fork can only be slender and light because it carries no bending loads at the dropout. With disc brakes forks would require a substantial increase in cross section (and weight) and brake would be heavier".

This is true, fork legs need to be stronger but not by a huge deal, this does not contradict the notion that larger rotors put less stress on the fork. Remember that the calipers and their mounting brackets fit to standardised positions on the fork less. I.S.O or Post Mount etc.. So its not like the caliper is placed in the centre of the fork leg. But the few MM of position adjustment the mounting hardware allows changes the leverage it has against the hub.

Also dont forget that people fit heavily loaded lowrider racks bolted around the centre of the fork leg (the weakest point).
And they do sometimes fail when the rack is poorly designed without a front brace like this;

http://forum.ctc.org.uk/download/file.php?id=11493&sid=231cae45a74dd452e730fa55cd7f437d&mode=view

But if they have the front brace the forks never break.

http://www.flickr.com/photos/mmeiser2/3056988352/