Size

When confronting an enemy in another size range, the following applies:

• The smaller being's attack:
  • Go with a difficulty modifier of -2 (all types of attack, except Grapple, that gets a +2 instead).
  • Damage rolls go with a -2 modifier (hand-to-hand attacks)
• The larger being's attack:
  • Go with a difficulty modifier of +2 (all types of attack).
  • Damage rolls go with a +2 modifier (all hand-to-hand weapons and natural weapons).

Among humans this is not very common, consider weight more than height.
This tends to be more common when fighting beasts.