It's not that Ms. Lavigne's arms are too wide for the mag — it's that her torso is not curvy enough. So they scooped out some vital organs over there on the left and created a tiny waist.
Of course, directly under her boobs is not really where Avril's waist is located. Her shape was exaggerated— into a severe hourglass — by slicing flesh off of her arm. The shot below looks slightly more normal.
But heaven forbid dudes see what an actual slender body looks like! Better save them the terror and just chop the lady's arm off.