Mannerism art was not always proportional. We can identify Mannerism in the Madonna of Long Neck by Mary's long neck and extra wide hips. Jesus also looks very unrealistic and un-proportional. Jesus, painted on Mary's lap, is much larger looking than an infant. Mannerism pieces seemed to go over the top of what was typical. Artists of this time would take what was normal and expected and push the limits just a little further. For example, in the Madonna of Long Neck, there is an "angel" with much of her leg exposed, which was very revealing for that time.
Mannerism works seem to be a little more exaggerated with the human body and it's positions especially. Michelangelo's art work during the Renaissance time also began to exaggerate some of the human body (especially the muscles). It wasn't until the time of Mannerism though, that Michelangelo really exaggerated the muscles of the human body.