Blender UV map unwraps weird? I'm working on a model and I've noticed that when I look at others, they don't seem to end up with the same "mess" that I get and I would like to know if there's a way to avoid getting this result? 

I don't mind having to fix this manually but I'm mainly curious as to why this happens. The model I'm making is a humanoid creature that's thin, tall and disproportionate. 



Again, I don't mind having to undo this and I'm mainly curious as to why this happens every time I unwrap a model.
This happens, if 


you change a model that was already unwrapped by adding geometry.
you join separate objects into one mesh while not all of them were unwrapped.
you join separate uv-unwrapped objects into one mesh, without adding seams between them. 


It might also happen, if you join two separate uv-unwrapped objects with unapplied scales.