Oddly enough, according to the article, they specced the exact amount of blur first, and then figured out the pixel resolution required to look exactly like the original picture blurred by that amount, and then the JPEG quality factor they could get away with.

If that's really what they did, it's weird for a few reasons, in an amusing but not ultimately that consequential way. First, there's a complex interplay between resolution and quality (Q) when it comes to how "good" a JPEG looks. For certain combinations of resolution, Q, and size-on-the-screen, turning the resolution up and the Q down actually makes the image look better, but if you go farther, it looks worse -- and that's when you aren't blurring the crap out of the entire thing anyway. Second of all, why does it matter if the final image has good "fidelity" to a blurry version of the photo that wasn't downsampled? The user doesn't know what the blurry image is "supposed" to look like, and yet the article implies they maxed out on information needed to reconstruct the true blurry image exactly. Oh, and third, there's presumably a range of acceptable blur radiuses from a UX perspective, while the 200 bytes is a hard limit, so choosing an exact blur radius and then feeding it forward through the rest of the design process doesn't completely make sense.

Because it's the answer to life, the universe and everything? :)