I can also feel my chicken's breastbone. They run around by backyard. All are very healthy, active and have no problem laying. The only way I understand they can gain weight is through confinement not a change in feed. I also do not consider a slim, not starving, chicken a problem. Human and animal population and laboratory studies have shown that the slimest individuals live the longest and have the least health problems. As long as all their nutritional needs are met, the chickens should be fine.
I am leary about giving my chickens more protein or fat than they need. Extra protein does not lead to more weight gain or muscle gain for that matter. (Only stress to muscles cells during excercise casuses muscle mass to expand.) It has the same amount of calories per weight as carbohydrates. Animals have a set amount, not percentage, they need each day to build muscle and repair tissue. Once this need is met, the nitrogen is stripped from extra amino acids and used as fuel. Fat does have more calories per gram. However, I would be leary of adding more to their diet to gain weight. Too much fat causes liver problems. It also has been shown to increase cancer risk in rats, mice and humans. Excess intake of certain amino acids, in particular methionine, have also been shown to increase cell aging in rodents, flies and primates. Since the aging process of animal cells are fairly similar, excess protein should also be avoided. Complex carbohydrates are pretty benign and the preferred fuel choice for most animals. However, they are preferred because they are easily broken down into energy. Most of it is lost to heat through exercise or to keep warm. So, more carbohydrates may not be that helpful.
Sorry for such a long reply. Hope it is healpful.
edited for typos