No, it's not due to them coming out of the pool, nothing like that. It's in fact, due to some smart coding by Stippi, who developed the Haiku Vector Icon Format, HVIF for short. In his most recently article about it, Stephan discusses the secrets behind HVIF which help make the icons so small and fast to draw.

The topics discusses are Coordinates, Paths, Flags, Matrices and Styles, giving helpful explanations and tips for each one. Anyone wanting to make icons for Haiku (but not exclusively) should read, so head over and enjoy.