By "twitter size" are you implying it's... small?

Twitter is the 4th most visited site in the world.

https://en.wikipedia.org/wiki/List_of_most_visited_websites

But it’s basically just simple data moving, it should be rather trivial in terms of compute power needed.

There is nothing simple in data moving. Problem is that it can be abstracted away so you do not have to worry about it. Out of sight out of mind. Example - majority of the CPU area is devoted to data moving problem (cache, branch prediction, out of order execution, register renaming). The same is happening on other levels of the stack (OS, network and so on).

They should have probably written it in Assembly.

It sounds high for sure.

But they do serve videos, which possibly they have to transcode or otherwise process on import (?) They probably also have some filters for pictures to remove porn and stuff like that.

This can amount to serious CPU if you have millions of users uploading that stuff daily.

A pure text-based Twitter would be much smaller.

There would be better EC2 instances (or AWS services) for that though.