Pixoto, Why Do You Destroy Photos?

I routinely submit my photos to both 500px and Pixoto, mainly for my own smug satisfaction of racking up high image scores on both sites.  Earlier today, I uploaded my recent "Merge" photo to both sites.  Afterwards, while viewing their scoring progress, I noticed that while the 500px copy of the image is as crisp as it was on my own site, the Pixoto version was apparently down-rezzed.  What's going on, Pixoto? Here are screencaps:

500px:

Pixoto: