This is because of the way tcp congestion control works. In order to achieve higher speeds, tcp has a "window" that grows, essentially the amount of data in fli

This is because of the way tcp congestion control works. In order to achieve higher speeds, tcp has a "window" that grows, essentially the amount of data in flight, that has not yet been acknowledged by your client. That window doesn't open very wide in the 100kb test because it happens so quickly.