> In total the thickness went down from 7 to 6 pixels, which is a 14% decrease, making it 14% more likely to miss it.

But also a 14% higher chance that you won't hit it by accident.

This is not a situation where bigger is simply better. If the thickness was 50 pixels, that would make it pretty much impossible to not resize the windows. I am one of those who believe that there are still people at Apple who care deeply about user interfaces. Given the amount of attention paid to the regions for resizing by dragging the corner, I actually assume that they also took a second look a dragging the edges, and concluded that 6 pixels was better than 7.