It's probably placed at an angle - the top being further away than the bottom. That means reflection points towards the upper end of the mirror are further away from each other, while points on the lower end will be closer - effectively stretching out the upper part of the image and compressing the lower part. You'll appear to have a longer and slimmer face, slimmer mid section and shorter legs. So yeah she is right regarding the mirror tricks, but the problem is that the fatter version is the truth.