I'm not familiar with 500pix or how it handles linking/sharing. The two indeed look dramatically different, but when I view at 100% and turn off browser zooming in Firefox, the image is small, but much sharper. Check to see if your own browser is scaling images up. Sometimes it can make them very blurry.
Here on POTN I am able to view 100% what 500pix is letting us display, and download that to inspect, and it's only a 130kb jpeg @ 900x615 pixels. Very small.
Is it safe to assume that the image that 500pix is showing is of larger files size and resolution?
My guess is that some setting on your host is only allowing a small compressed image to be shared with POTN.