The 50 bytes figure represents a compressed response. (We actually write all of our pages compressed to disk before serving them -- nothing is served dynamically. But that's for another post...)
The example below isn't the most scientific, but should give you a rough idea.
Test index page with different static URLs:
URLs as 4cdn.org -- 23261 bytes compressed
URLs as 4chan-cdn.org -- 23311 bytes compressed
URLs as 4chan.org (control) -- 23278 bytes compressed
Can someone with more ZIP-algorithm knowledge than me explain why a difference in word length of 5 characters can result in a difference of compressed result length of 50 characters? I mean, a reference to a 1000-character word takes as much space as a reference to a 1-character word?
The example below isn't the most scientific, but should give you a rough idea.