You're right, hellfire - short prods take large amounts of set.  But that is beside the point, and medieval crossbows were made to these very short specifications.  So they are authentic, and undoubtedly powerful.  The power comes, in a crossbow, not from efficiency of energy use.  When you can make a piece of kit like a goats foot lever or a crannequin to span your bow, you can take the draw weight up to about 200 pounds or more.  Who needs perfect efficiency if the thing is authentic, looks good, and shoots bolts well?  I'm not saying the European medieval crossbow prods were efficient... just that I wouldn't like to face one without a pavise.  Or a gun.
  I assume you've never made a crossbow?  They are good fun.