Myself I feel it is a combination of things.. The first surely being the genetics.. Without good genes then it wouldn't matter much.. Next would most likely be the food source. As here in my part of the state there are few large gardens and the birds tend to weigh less than the southern parts of the state where there are more farms. The tail I believe would depend on it's environment. Meaning is he apt to be in more open country or is it more of a wooded area?

Now this is just in my opinion for what ever it is worth and not any kind of a survey or study on this subject.