Used it on my iPad yesterday, and did notice the less-than desirable viewport width. Google seems to have been inspired by webOS's Mail app, but has forgotten that the quick-view could be expanded by pushing the inbox view away so that only the message is in full view.
Also, the grey borders are just a case of prioritizing form over function.
Just testing on my phone, I get the feeling the newsletter this OP has screenshotted has set it's own padding/margins as well, exacerbating the problem. Still, it should have been quite clear when they were designing it that two margins for a 240px width screen is overkill.
However, the inbox screen isn't so spaced out, and relatively condensed. Thankfully
Also, the grey borders are just a case of prioritizing form over function.