Why would a mirror begin to magnify things?

Chris - All amazing and fascinating stuff. Well, it's time for a change of gear and to open our mail bag for the week. What have we got in, Hannah? Hannah - Well, I think Chris, this is a question for you. Don McKenzie has been in touch saying, "Why would a mirror begin to magnify things?" So apparently, he's got a small handheld mirror, which he and his wife have had for 10 years and one day, his wife used it and it suddenly magnifies like a shaving mirror. A few days later, they picked up the mirror and it was back to normal. Is there an explanation for this change in magnification of mirrors or was that just all in his head? Chris - No, I think it probably is real. I suspect what has happened is that the mirror, which is mounted on a back plate. It's usually a reflective layer mounted on some kind of back plate. I suspect that probably heat or cold has caused the mirror to change its shape very subtly and it's become like those fun mirrors that you see at the fair ground. You know, the ones which are ripply shaped and they make your body have all kinds of funny proportions. Effectively, if the mirror becomes slightly curved, slightly concave, then it will have the effect of focusing the light which is coming back to you almost like a lens and the light will then begin to spread out again on its way to your eye, making what you see in the mirror look bigger than it really is. That probably happened one day and then the next day, the mirror had reverted to its normal shape again. Perhaps when it cooled down or warmed up. And that's why it went back to normal. Sound plausible to you?