The link is using ColorBox to render the "pop up" window, via the cblink CSS class. By default, the cblink class tells ColorBox to render as 85% height and width (you can see what it's set to by viewing your page source):

$('a.cblink').colorbox({width:'85%', height:'85%', iframe:true});

So my guess is that someone wanted a smaller ColorBox popup in another part of the site and overrode the cblink rendering globally (by using parameters on the ScriptLoader control in the Layout.Master of the skin).

So the first thing you should probably do is remove any ColorBox size overrides in Layout.master. Then, if you need a separate type of ColorBox link for specific purposes, do that by using a different CSS class and corresponding ColorBox initialization in Layout.Master. I've used this technique for custom features where I want an informational ColorBox popup to tailor itself to the content size, which is the default behavior. For example:

<script type="text/javascript" >$('a.cbinfolink').colorbox();</script>

I hope that helps,
Jamie