I got one on my Pixel 4 just now and it's 2.4 MB. I went to System Update and the message was there that an update was waiting to be downloaded. It only takes a few minutes to download but then it has to optimize apps. It's doing that now and I have no idea how long it will take. Optimizing apps is taking as long as it did with my Android 10 download and I have only 85 apps total. Some have a lot more than that.
Okay, about 7 minutes to optimize apps. That's it. Works fine. I checked to see if I could use Gestures with Nova Launcher and I can't. Gestures is still blacked out but I think I'm happier with two buttons.
The info screen for the security update says "...fixes critical bugs and improves the performance and stability of your device." Me? I've had no critical bugs or stability problems.
I checked and it still shows my latest security update as September 5th so apparently neither Google nor I consider this a new version of Android 10. A minor bug fix at best.