They're living together? A couple of thoughts...Did you get one more recently than the other, have they been fed / are being fed on the same schedule? If you just got the smaller one from a place that feeds less often, than I'd say the size diff. is normal. But if you've had them the same amount of time, and the smaller one isn't feeding as well, or just isn't growing, I would def. try seperating...the smaller may be stressed, which could cause it to not grow as well as the ghost.