Simply wrap it in another div with a fluid width, such as
width: 100%, like so:
<div style="width: 100%"> <div class="g-comments" data-href="example.com/page-permalink/" data-width="600" data-first_party_property="BLOGGER" data-view_type="FILTERED_POSTMOD"> </div> </div>
When set this way, that width of 600 appears to become the maximum width, instead. Now it scales down perfectly!
Note: As of writing, this whole Google+ comments thing is an unofficial feature (it was lifted from Blogger, the only place that officially has it), so it could break practically any time — and this tweak may not function the next time you get up from bed. Keep an eye on it… At least until Google releases an official API.