Starting with 16.0.13707.20008 (ProPlus, Win10 Preview) earlier this month, build updates involve much more downloading than they ever had before in my years of updating Insider builds.
By the time this week's update rolled around (the third time in a row this has happened), I took the time to measure what was downloaded: around 3.4GB. That's basically the entire client, as if I'd installed from scratch. What happened to differential updating?
I happen to have a way to see what the bandwidth was historically: roughly half, consistently.
I tested on another PC (last week's build to this week's, as always): also about twice what it should be.
The "Host Process for Windows Services," which is what does the downloading, just isn't as efficient as it used to be, but why?