Actually it's real late and spent way too Long on this today..

If you wanna have a go basically you need to add a div on the outside of the a tag give if an id say productimagefocus and an inline style with height and width defined by
The MEDIUM width and height define.

Once you do that then apply the css
#productimagefocus img {
Max-width: 100%;
}

In theory that should work!