Played with proxy and checked it myself. And I can confirm that it does work.
It's a bit hacky though. You need to right-click-open-file cover image and then add couple of zeroes in file location to see full sized covers.
For example, you would just need to add two zeroes at the end of this link to get 1400 x 900 cover.
https://books.google.com/books/conte...VH0C?fife=w300
Quite awesome results in certain cases: