I would move the smaller discus to another tank if you can. I know from experience that litle discus will not grow as big as thier adult tank mates. The reasoning behind this is because of a hormone that is excreted by larger adult discus. If the newbie is still getting picked on try uping the WCs and uping the amount of food that you are feeding or the frequence. I had one ******* discus that picked on everyone for a long time, 6+months. I just stuck to a stricked WC schedule and tried to rearrange here and there.
IF this continues, you might want to try taking out all the discus for a couple hours then put them back. For some reason this causes discus to re-evaluate their other tank mates. Not sure why, got that tip from my breeder friend.