Well.. I've been on there for quite some time, granted it's been not mega active but here is a rundown of how much it took until now: there is the main sigchain database, which stores all the messages (following, posts, ....) which is now 150megs in size and there is the blobs (binary attachments like images) which is about 500megs in size. YMMV depending on how many catpictures your friend share ofc.
The flipside to your remark is, that it is fully offline capable and I'm perfectly happy with that. Also: contrast it with how much space a thunderbird profile takes up.
How would that change if you had, say, 5,000 friends – the fb limit, which some people do reach – who were posting multimedia content multiple times a day (which happens on fb)?
Is the protocol set up in such a way as to enable easy, automatic deletion of old data from local devices, while still storing them for easy search/scroll-based access on the Pub servers?
That's going to be a bigger problem on mobile than it is on desktop (I mean, not literally those amounts, but the amounts you'd get from a busier feed).
The flipside to your remark is, that it is fully offline capable and I'm perfectly happy with that. Also: contrast it with how much space a thunderbird profile takes up.