This is very common. You just got it, right? Well, it is just becoming more and more happy in your tank, and so is expanding more and more to it's actual size. Almost every coral I've seen for sale isn't expanded to it's full size, because of the poor conditions and because they have been moved around so much. They sorta shrink down when they are stressed. So, now that yours is finally in a good peaceful location, it is feeling comfortable enough to expand back to it's true size. So, while it probably doesn't mean your coral is growing, it does mean it is happy and that you bought a bigger coral than you thought you did, which is a good thing. Congrats, hope it keeps doing well. I've been wanting to get one of those for a while.