Question:

Why is the file size of Tumblr 'raw' PNG files sometimes lower than that of their 1280 versions? For instance, post #1169845 vs post #1253844.

I decided to upload the latter anyways since the image clarity zoomed in to the same resolution was superior, without any visible artifacting, but I'd like to hear some thoughts on what could be causing this. Does Tumblr somehow increase file size when compressing images to the 1280 limit, and if so, why?