In the rulebook of ToI it states on p.8:

Large monsters (monsters that occupy more than one space) are difficult to affect with the Knockback ability. For each space beyond the first that the monster occupies, it is moved one less space by knockback. For example, an ogre occupies four spaces, so it moves three fewer spaces when affected by knockback. A dragon, on the other hand, occupies six spaces, so it would move five fewer spaces.

I don't get this. Knockback allows you to move the target op to 3 spaces in any direction. Can an ogre or dragon even be moved then?

There are a few (maybe only one) weapons that convert surges into ranks of Knockback, hence that clarification.

Found it: Mace of Kellos

So this means with normal knockback ability you can't knock monsters with a base of four tiles of more back?

Yup, normal Knockback only works on 1 and 2 square monsters, one of the few realistic things in Descent IMO