Had a look and I find one thing suspicious....whatever it is on the wall to the left of the mirror (looks like a big calendar, or painting). If you look very carefully, you will see waht could be the same pattern on the dress. Now, given that the mirror is angled, it could be that this is just a reflection of what they would be seeing in real life as it were. I know some mirrors are capable of increasing size, elongation, etc, etc.
I think it might have just been a coincidence that was noticed, and there to be taken advantage of....and was