There is no such thing as being big boned or heavy set.
Oh yes there is. Medically, bodies can be classified as endomorphic, mesomorphic and ectomorphic, and everybody falls into one of those classifications (I reckon I'm endomorphic).
However, whichever one, you are quite right that you can put on weight and lose it (although the extra weight could be fat or muscle), by eating more/less and by expending/conserving energy.
I meant skeletal wise. All human skeletons are pretty much the same. I agree re muscle mass though.
Yes, but skeletons come in different shapes too - some wider than others, some taller than others, and so on.