OK, now I am pretty sure that Google changed something (I hate it when this happens and the developers don't get to know). In an earlier version I had patched an issue with Colorbox, which was displaying images in their full size, beyond the size of the window. The patch was done to scale the image down appropriately to make it fit. I didn't have to fix Fancybox because it did not have any such problem. But now I see that even Colorbox is showing me a smaller image that is 512px wide. So it is definitely a Google API change.
I will look into this for the next release.