It just seems like a dumb thing that serves no purpose that wasn't thought through and does more harm than good and probably popularized by some image upload web 2.0 service just to look cool.
I mean, I could understand if .jpg-large were actually so large that many normal programs have difficulty with them... but how is 1024x768 "large" by any means of the word, when we've had monitors at that resolution since 1999 at least (I think that was the exact resolution of the CRT monitor my family got about mid-1999). If the resolution of the images were greater than 4096x2048 or something, I could *kinda* see that there *maybe* might be a need for it.
It would make more sense if large files could be better compressed, and faster, at a higher quality with a newer algorithm, so .jpg supports another algorithm that most earlier programs aren't familiar with, so to preserve backward compatibility, .jpgs using the new algorithm use a different extension but still have the same image format... but I'm hypothesizing here, Wikipedia doesn't have any information, and my googling turned up too many irrelevant results.
So my question is, what is the purpose of this extension? If it has no purpose, who is responsible for this travesty?