I realize this first came up when the Voyage first came out, and I discussed it here:
https://bugs.launchpad.net/calibre/+bug/1385298
At the time, it turned out to be an add-in that apparently disabled the new thumbnail height. This time, I tried disabling some add-ins and it didn't make a difference. I am running the most current version 2.50.1 64 bit. It was a slightly older version when I first got a small thumbnail yesterday, and I updated Caliber and still got a small thumbnail.
I know it sounds crazy. Why would it happen, a year after the code was changed for the Voyage to send a 470px thumbnail, would it suddenly send a smaller thumbnail? But it is. I watch the thumbnail appear in Kindle\system\thumbnails, and it is 330.