This is only problem on fast and capped connections which I would guess are rare. On slow connection (and proper browser) you see that it's downloading data, how much, how fast and you can stop it.
4g is fairly fast and generally capped. That is hardly rare.
Oh, you are right. I was under the impression that 4G is something like 2-3 Mbps, which would make the caps somewhat proportional and hard to pass by accident. But I just tried speedtest.net (app) and download speed was almost 40 Mbps. I really had no idea.
So maybe browsers should offer a setting to cap the data retrieved from each individual domain on metered connections, unless instructed otherwise on a domain-by-domain basis?