Well I guess this is due to authorization restriction.
The reason why your code, Carlos and myself get a 4K file is because we have not the rights to access the requested file.
I guess you've used a sort of login procedure to access the site with your browser, or are on a trusted network allowing the site to use your NT tokens.
What seems to confirm this hypothesis is that when I change the extension of the GetBinary.aspx file to jpeg I obtain a kind of "forbidden" icon.
So you may have to do more work from your code to authenticate.
EDIT: using the 'NIX file utility with Cygwin gives this :
JPEG image data, JFIF standard 1.01
So what you get is indeed a JPEG image.