Not a lot of pruning. Incidentally, writing the same thing in Go (for example) yields more or less the same binary size.
You should see how quickly the binary size grows as you add more of your code (answer: not much) and what the alternative is: a txt file containing "hello world" and a statically built nginx server.
They list the binary size as 27MB where a similar Go binary is just under 5MB. So I'm not sure where you got your numbers, but they're wrong. Also you were responding to a comment about the runtime resident memory (RSS) of the app being 40MB, not about the file size.
You should see how quickly the binary size grows as you add more of your code (answer: not much) and what the alternative is: a txt file containing "hello world" and a statically built nginx server.