In one of the linked prior blog posts[2] of his he linked to in the beginning of that article, he said this:
Using the Hilbert curve layout leads to a very simple compressed representation: having sorted the u64 values it uses to represent edges, one can simply delta-encode them (writing the differences between adjacent values). Because the Hilbert curve teases out locality, the gaps are often quite small, and therefore compress nicely. When I combined a variable-length integer encoding with gzip, the resulting file was only 45GB; about 2.8 bits per edge). Decompressed (with just the variable-length delta encoding), it is 154GB, which is the representation I used to get the measurements above.
He then goes on in his posts to comically trounce some large distributed systems.
1: https://news.ycombinator.com/item?id=15332051
2: https://github.com/frankmcsherry/blog/blob/master/posts/2015...