Correct me if I'm wrong, but NV stacks aside, MF doesn't actually increase the number of items dropped. The only thing MF does is it increases the chance that a dropped white item becomes blue, a blue becomes yellow etc.
For example, an elite pack has a 4% chance to drop a rare with 0% MF. That chance becomes 8% with 100% MF and that's that. (All numbers made up).
That's how it worked in D2 and I believe Blizzard mentioned MF works the same way in D3.