I'd guess there is some offset power needed for keeping a "line" open. Like, 200 kB/s is not twice the power of 100?

When looking at the power consumption across the whole network path and not just a single link, most of the power draw is probably baseline static power costs of keeping all the routers and switches running. Which means that judging the impact of a download in terms of Watts per MB/s is a pretty bad way of analyzing this.