WhatsApp fills your phone without asking. Here's how to stop it
Tips
If you're in a family group, you know how it goes: your cousin's holiday photos, the morning memes, the same video forwarded by three people. By default, WhatsApp downloads all of it and puts it in your gallery. After a year, that easily adds up to several gigabytes.
Turn off auto-download
In WhatsApp, go to Settings, then Storage and data. You'll find "Media auto-download", with three lines: mobile data, Wi-Fi, roaming. For each one, untick at least videos and documents. You can leave photos on for Wi-Fi if you want.
You don't miss anything: the media shows up blurred in the chat, you tap it and it downloads. You just get to choose.
Keep media out of your gallery
Second setting, in Settings, then Chats: "Media visibility". Turned off, received photos no longer show up in your gallery. They stay in WhatsApp. You can also set it group by group, from the group info, if you want to keep the photos from just one chat.
Catch up on the backlog
For what's already there, WhatsApp has a tool: Settings, Storage and data, Manage storage. It shows files forwarded many times and files larger than 5 MB. That's often where the heaviest videos hide.
One trap remains: when you save a received photo, it can end up existing twice, once in the WhatsApp folder and once in your camera or downloads folder. Those are exact copies, the kind of duplicate you can delete without regret.