I think this is going to have to be a necessary rule.<br/><br/><strong>Size is Power</strong><br/>An important characteristic is that there is a natural hierarchy basicly based on mass and relative strength.  Species of a lower rank on the hierarchy find it very difficult to actually inflict stress on creatures from a higher rank.  This goes in reverse, though, as those of a higher rank find it very difficult to directly attack creatures of a lower rank but do so for more damage.  There is a penalty of -1 for every rank of difference between species.  For example, Cats and are at -2 in any encounter with a Human when trying to inflict damage (though +2 to actual attack), and -3 in any encounter with Anything Bigger(and have a +3 bonus on attack).  A Human would conversely be at -2 to attack that same cat (though do +2 damage) and Anything Bitter would be at -3 on that same attack roll (and do +3 damage).<br/><br/><strong>Hierarchy by Size Tier</strong><br/><br/><strong>3rd    Anything Bigger Than Human</strong><br/><strong>2nd   Humans</strong><br/><strong>1st    Most Dogs and Small Children</strong><br/><strong>  0     Cats</strong><br/><strong>-1st   Anything Smaller</strong><br/><strong>-2nd  Insects</strong><br/><br/>One of the 'Create an Advantage' actions should be to be able to remove this advantage temporarily.  For example one could remove these advantages while fighting a human by jumping on his head and then going to town.  Hurts... for real... but they have an easier chance attacking you because you have removed the defensive side of the size hierarchy as well so best do these kind of manuvers from behind or in darkness, etc.