You are actually right, I did notice this. Although, maybe its the other way around. What I remembered was the image being much clearer, rather than bigger. It is possible that the other sites that it was uploaded on have the correct size, but slightly lesser quality.

If that isn't the case, and it is just in the quality, is Waifu2X the best option available for this sort of thing? Or is there another alternative?

I'm still looking to see about the original illustrator.