Because when it's NSFW it's probably an image or video which uses more bandwidth than text does, so if you're scrolling all those images and previews need to be downloaded and if it's prefetching more than just a thumbnail while you scroll you can end up with a lot of data transfer.
Not that far off from mine! Interestingly, my Sync used almost as much as Meet (which I use daily for video calls).
I suspect it's mainly multimedia data, like high-res images. Maybe Lemmy serves higher-res images compared to Reddit? I also recall seeing some discussion somewhere were Lemmy instance admins were turning off server-side image/thumbnail caching, so as a result you could be directly pulling the high-res images by simply scroll thru your feed? This is just a guess though.
My guess: Reddit was serving not high res pictures (thumbnails) while scrolling and only gave them when opening.
Instances would need to create a thumbnail and the actual video. This would require a smal-ish CDN and processing power + storage to save the multiple versions to, right?
Idk about NSFW but arent they just hidden? To my knowledge at least Sync only shows "NSFW" and a blank single-colored post for an NSFW post.
I'm thinking similar. I have preloading hires images off and gifs set to WiFi only, but when I go to view an image it is already there loaded. Guess I'll have to turn off preloading previews and thumbnails also. I don't watch video here unless by accident.
I think it might just be a bug or weird optimization issues.
And I'm pretty sure OP has that much data use because they're on lemmynsfw. Not only are they consuming a lot of media content, lemmynsfw admins have been turning off caching for images so he's fetching the linked media every time. I only have 1.06GB used since August 2