pecking could be caused by so many factors, first and foremost I would suggest that you check that chicken over carefully to be sure there is nothing visibly or physically wrong with her. Second I would remove the top hen for a few days and see if things calm down and then put her back in, if she goes for the attack once more then remove her again. Until she gets the idea really. breaking up the pecking attack when it happens is also a good start.
Check your lighting, what have you got in there now? if anything? a red light has been said to reduce pecking, but I dunno about that, I use a heat lamp in the winter and well my hens still pecked at each other if their combs got injured, so I didn't really see a difference.
Of course, square footage of your coop and run if you have one, if their accommodations are too cramped they will peck each other, I am not sure what your set up is like.
lastly, if she is really small compared to the rest she is obviously the one at the bottom of the pecking order, no pun intended, you do need to focus on her getting more protein and more attention so she can gain some weight and get a bit healthier, a scrawny chicken is not going ot be in the healthiest of states and can't possibly stand up for herself against bigger chickens.
I have always removed the top bullies in my coop for a day or so whenever I have seem them misbehave or go after another hen...and it has always worked. Now I haven't had to always separate them, but just being picked up in the moment and carried around under my arm has proven to them they are not top hen, I am!! and that tends to settle them up.
adding a second feeder and waterer in there might also help the hen being picked on.
Good luck, keep us posted,
Ema