Pretty sure a nontrivial percentage of peoples smartphones have that many thumbnails for the photo gallery app alone.

Anybody who has had a smartphone for a decade likely has at minimum 10k photos in their cloud locker with local thumbnail.

Ok, so add some more megabytes to that. Most people don't have that much microblogging data.

I actually think photos could potentially add up to quite a lot!

Sure, they could. Most people don't post tons of hi res photos. But I'm sure there are ways you could optimize to not have all the content on local device, if it's such a big deal. But this is a really strange point to me to be hung up on.

I think you’d be surprised at the number of photos posted, but also many people post tons of gifs (especially reaction gifs) which are fairly large.