Frankly it is an arbitrary number, mostly. I started with the goal of having no more than 7 functions, because that's the number of functions the first version

Frankly it is an arbitrary number, mostly. I started with the goal of having no more than 7 functions, because that's the number of functions the first version had. It didn't take long for me to figure out that this wasn't enough for every-day use. My next goal was to be a number of magnitude smaller than jQuery, and thus 3.3kB. That was a lot better, but still not enough to be a viable alternative. So I settled with 4kB. While 4kB may fit well in certain packet sizes, this does not help with HTTP headers that can easily have 100 bytes or more.