...as to your second question...I can't tell much from that photo as she is standing in an odd position as to her overall health and conformity...but it probably is just a large crop from a big dinner. You can see how the crop works here: http://www.mypetchicken.com/backyard-chickens/chicken-help/My-chicken-has-a-lump-or-tumor-on-her-chest-what-H88.aspx
I personally have never heard of any kind of big breast disease...they can get impacted crops, which you may wish to check her for. An impacted crop is just that, a crop that is not clearing food further along into the digestive tract because it has become "clogged." Giving the bird some olive oil can help smooth things along. Make sure they have access to grit to help gind the food up in the crop (it's purpose). If you feel it is an issue, post on the Emergency/Disease forum and a lot of people can help you there.
Hybrids, which is what your bird would be (a mixed breed) typically do not succumb to the odd genetic diseases as pure breeds do. Some rare breeds suffer from too much in-breeding as most of the lines are closely related and do have trouble with anatomical or immune issues. Usually it shows up first in failure to hatch chicks, but there are Veterinary books full of odd stuff I've yet to learn.
Meet broilers succumb to a number of genetic and growth abnormalities, one of which is Green Muscle Disease which is atrophy and necrosis of the breast muscle, found at processing. Green Muscle Disease is thought to be caused by the broilers fast rate of growth and overall heaviness causing restriction of blood to these breast muscles. Your bird won't have that as that is specific to meat broilers.