Head tube length is pretty much determined by what the other specs of the bike need to be. It's not like they design the bike and say, "we want this one to have a 130mm head tube." Head tube length is a mostly useless measurement, in that it doesn't tell you any exact information about the rest of the frame.
Generally, as the frame size goes up the head tube gets longer. This is most easily seen comparing two bikes with level top tubes (see pictures.) Things get more confusing with sloping top tubes, because the head tube needs to be longer to meet up with the higher top tube.
