This is a pretty simple answer. The range of a typical aoe spell is 10 yds from a center point. FnB and SoC were hitting with a range of 15 yds from a center point. Thus they could hit more things that were more loosely pack than all other aoes. By reducing the size to that of other aoes they fix that problem. MF affects all our aoe spells not just the ones you mentioned.With that in mind I honestly dont disapprove of the nerfs to FnB and SoC, but I do wonder why Immolation Aura/Hellfire and RoF with MF didn't get touched.