My image keeps getting zoomed in using parallax scrolling.

Question

Any help will be gladly appreciated, I am trying to do parallax scrolling, but my image keeps getting zoomed in.

Parallax: Parallax http://www.shck.mx/tests/parallax.png

No-Parallax: No-Parallax http://www.shck.mx/tests/noparallax.png

I am using this CSS code:

#about {
background: url(../images/nevsky.jpg) 50% 0 no-repeat fixed; 
height: 400px;  
margin: 0px auto; margin-left: -80px; margin-top: 0px; 
width: 120%; 
max-width: 1320px; 
position: relative; 
box-shadow: 0 0 50px rgba(0,0,0,0.8);
box-sizing: border-box;
}