Fighting creatures smaller than your character can be tricky (see the section on Size Classes in Other Rules for details).
The Fight Small skill reduces the difficulty due to different size classes by one-half of the character's skill level in the skill, to a minimum of zero (a character should never get a bonus against smaller opponents due to this skill).
In the example under Size Classes, Rutger had a difficulty of 30 to attack the bird. If Rutger had the Fight Small skill at a skill level of 50, his difficulty would only be 5(=30-50/2=30-25).