Where are you noticing the lower resolution thumbnails?
Here's a good example right here:
Clicking on the quick build list on a planet. See below for image >
BTW, the memory difference between the two is approximately 170 megabytes of memory in a typical large galaxy game.
I can see where that might be a save for low end systems. But i can only speak for me, i have a 64-bit system and 4GB of RAM, so for me 170Mb is practically inconsequential. I'd much rather the larger thumbnails so you don't get all that pixilation. I'm all about the eye candy (i designed the High-Res graphics mod which i’ll be updating for Twilight, if anyone’s interested in hearing).
Perhaps it could be made an option in settings? "Use Low-Res thumbnails"?