I finally cornered the WhatsApp bug: It starts writing to disk, at a rate of 3MB per second, when you post to somebody a link to a video. Yes, a URL, not the video itself. Every sane programmer would say "oh, a URL is just text, that's 50 bytes or so", but nope, WhatsApp TheBug starts writing 3MB per second.
It stops, btw, if you look at a different profile, and restarts when you come back to the profile where you posted the URL. So I thought "maybe I'll just delete that post", but nope, it continues to write 3MB per second, even after closing and restarting WhatsApp. This is sick, folks.