You also have to consider a dramatic increase in what JS does in addition to there being 10^10 more JS in the first place. You didn't really see JS libraries being used on every site (especially staples like jquery and angular, which combined add a few megs to pageloads). The use cases of JS have shifted - many sites don't even render anything with JS turned off anymore.
1 billion = 10^9. If what you say is true, then the average length of JS code must have increased about 10^10, because despite increased bandwidth, most sites seem to run slower than they did 6 years ago. That is some pretty prodigious coding(8-))
Or maybe it's write delay caused by the NSA.