Given that the main rules take no account of the anatomical differences between the three player races, you can cover it with this:

Dwarfs add one misfortune die to an Running or Leaping based Athletics check. To counter this, the other races suffer a misfortune die penalty to physical checks when in cramped conditions (tunnels, sewers, etc.); Dwarfs don't.

excellent example of using the spirit of the rules to come up with your own houserule flavor.