The first compilation, which is 160 KB, is totally unoptimized, contains all kinds of debugging and checks. It's just supposed to be for yourself during development of the program.

Also there is some overhead every Nim program has. But if you get to bigger programs you'll see that Nim's binary size is just fine, for example a NES emulator is just 136 KB: http://hookrace.net/blog/porting-nes-go-nim/#comparison-of-g...