Feeling the keel is not an indication of an underweight chicken. The prominence of the keel is genetic, in a chicken where it is prominent genetically no amount of food will change that. The chicken does not naturally store a significant amount of fat in that area and the amount of meat they grow there is, again, genetic. You cannot force them to produce more breast meat with food -- not significantly enough to overcome genetics, anyway. That's why Cornish X are bred because they have the genetic predisposition to produce ample breast meat. If we could force that on any chicken with any genetics dual purpose meat flocks wouldn't be such a project.
Likewise, feeling "light" is not an indicator that there is a problem. Most chickens will feel "light" compared to what they look like they should weigh. Much of their mass is feathers. Feathers are light. Only very compact, meaty type chickens will weigh what they look like they should weigh.