In Bitcoin Wiki Weaknesses: Sybil attack they mention that:
"Bitcoin makes these attacks more difficult by only making an outbound connection to one IP address per /16 (x.y.0.0)".
Why would this make a Sybil attack more difficult? Is there a reason for choosing a /16 subnet or is it randomly chosen by the developers with no background? Wouldn't it be more secure to choose a smaller subnet?