I bought two purple tip anemones last week and mine does the same exact thing. It turn two times bigger when the lights go off.
On a different note, I really regret buying the aneomones. Now I'm super paranoid of them dying. Yes, they look like they're in good condition, but I hear anemones don't do well in captivity. And if they die, they're going to release their poison, possibly killing everthing in your tank. That's from what I've read at least. Not to mention the amonia spike. Be careful with them and keep a close eye on them. I don't think you want to go on vacation with anemones in your tank. Just in case the worst thing happens. You want to be around to immediately remove them from your tank. On that note, that's the last time I'm going to buy an anemone. A coral that resembles an anemone might be a better choice.
I don't mean to sound like a prick or anything, but that's what I hear a lot of people tell me.