Maybe is a stupid question but....
What is the diferent between medium (large or huge) creature and large or huge creature?
The tables give a DB bonus to medium creatures being attacked by large and huge creatures.
It gives and OB penalty to medium creatures attacking Large, Huge and Super Large cretures.
That is what those four lines of modifiers on the combat tables represent. Big things find it harder to hit small things and small things find it harder to hurt really big things.
The sizes are specified in Creatures & Treasures, in the description of each creature. The effects of size are described in the Giantism - Larger Than
Normal Creatures rules on page 139 of the RMC Creatures and Treasures.