That’s what the HTML code asks for. I have Chrome set to zoom a couple of levels most pages, which is why mine was upscaled. The aspect ratio difference is not huge; R(720)/R(700) = 0.996..., R(500)/R(700) = 1.003. I think the difference was that mine was upscaled and thus had a bit less local contrast (i.e., was a bit more blurry). I still find even the 500x257 one harder to see, but that could be expected as it’s simply smaller and thus has less info even with a perfect scaling algorithm.

I mean, one would expect a scaled image to be slightly harder to see clearly, but it's strange how much of a difference there is for seeing the fox without actually noticing any specific difference about the image. (I actually had to check the sizes with the HTML inspector, as the difference between 720 and 700px was too low to see clearly when switching tabs.)