Because a 230px thumbnail is hard to see and was unnecessarily small on most modern devices and screen resolutions. It's the reason we changed it in ApartmentGuide.com too.
"Google's stupid system reminded us to propagate a change we'd already made on another site for pure usability reasons" is not at all a bad thing but I think the general tenor of the responses here makes it clear that the blog post didn't make it obvious that was a reasonable tl;dr.
(I thought I'd read to the end but hadn't realised that was the tl;dr until I read your comments, FWIW)