Google AMP mostly just reduces the latency by pre-loading the pages from Google Search results. That has the side-effect of increasing the traffic on networks using Google Search when Google AMP results come up on the Google Search results, which they probably will given that using Google AMP will improve the site's Google Search ranking.
I haven't benchmarked it, so I can't actually speak from anything but wild speculation, but Google AMP might actually be making this problem worse.
I gave it a try. So I searched for "Trump" in Safari mobile, the result page loaded 2.5mb. It loaded a bit more when I clicked on an AMP article and kept loading more as I jump from article to article. It did not load any data without me switching to the next article.
After 10 articles, the transferred data was 28MB. I visited the Guardian's own website and it fetched 3MB.
So, It looks like Google does not preload a huge amount of data.
How are you able to look at how much data was transferred on Safari mobile? As far as I know you can't access dev tools on mobile. Did you just change the User Agent on your computer to simulate?
You can! You can use the desktop Safari to connect to your iPhone's Safari and access full developer tools.
Just connect your iPhone to a Mac, open Safari desktop and in the Develop menu, you will see your mobile phone. When you open a page on mobile Safari, you will be able to see it from the Develop menu and when you click on it, the full Safari developer tools will open in a new window. Works just like the regular developer tools.
I haven't benchmarked it, so I can't actually speak from anything but wild speculation, but Google AMP might actually be making this problem worse.