They would simply have additional file servers, could be any amount depending on how many video/images.
They would possibly have to be in separate datacenters though, since shipping large amounts of data over seas can be inefficient and possibly expensive.
I'm not sure at what point that starts to be a necessity, they're big but it's still a niche market. And their content is very sparsely visited (i.e. it wouldn't happen often that 80% of their audience would view a single question).
The stats that would be hit the most obviously would be bandwidth, followed by RAM.
Storage requirements would change of course, but
bandwidth would be absorbed by whatever CDN they're using, so traffic quantity stat would be different.
Also, with images/videos, they're now sending more URLs/embeds/etc. along with data, so more characters = more RAM for caching and/or serving up on web servers. In theory, shouldn't be much more of a hit.
