Unfortunately it's not at all normal, it's swollen to the extent that the bottom of the case is slightly warped and the macbook will no longer sit flat. It's also affected the trackpad which doesn't click very well any more. I noticed these things a while back but didn't realise that the battery was the cause until I opened it up a couple of days ago to clean it out (something I do once or twice a year).
Now awaiting a battery replacement kit from iFixit and not looking forward to removing the old one. Probably going to try the dental floss method someone mentioned above (very, very carefully and far away from anything flammable).