I would definitely start paying better attention to the smaller one. Raw egg yolks are good for energy and giving nutrients. Electrolyte water is also a good energy booster. I have a recipe for it if you'd like it.
Is the smaller chick not getting her fair share of food, lethargic, not doing well all around? Try sitting with them with your hand in their container for a little while and just spending time with them petting, feeding, holding, etc. Make sure your movements are slow, nothing quick and...