Aperture blade shape has to do with it also. Most of the wides have 7 blades but are curved, with the exception of the new 24 1.4L mkII it has 8 also curved.


Where it comes into play with your question is the shape of the out of focus elements. See Bryan's review of the 3 different 50mm lenses for great examples of this and aperture blade shape. The 1.2 and the 1.4 both have 8 blades but there is a significant difference in the bokeh.


Just like blurred out lights in the background everything out of focus gets smoothed and takes on the shape of the aperture blades. While not technical mumbo jumbo, the wider the aperture, the more space there is for things to get smoothed out, taking any fine hard edges away.