Its new to the tank isn't it?
It looks like its just doing alittle expansion contraction thing right now, though I have seen some RBTA's that regularly look very bulbous like in your first pic.
It also looks a little bleached out, I hope that you have sufficiant lighting for it to regain a healthy population of zooxanthellae.
Although BTA's can survive with little direct feedings I would suggest that you feed this one 2 or 3 times per week until it darkens up.
Thomas