Hmm I didn't think I would get so many responses.
That's probably what Neonivek intended. He mentioned that it could be based on size advantage, which means that a size 7 human could easily pull this off against a size 1 whatever
Half and half. Some attacks could only be done with a highly dense or large creature (Shockwave for example) even if the size difference between the two was immense. Also others, such as kicking the earth at the enemy (Excluding rock), could have an effect if there is a large size difference but would only have devistating power with legitimately large creatures.
What of uh... Collosi? Dragons? Titans? Other, such megabeasts.
Collosi are mindless and thus this list for the most part shouldn't affect them.
Dragons have Firebreath and can lift you up into the air and drop you to your doom.
As for a Hydra, one thing Id love to see them do is for each head to hold a different part of your body and for them to tear you appart.
An example being stomping on foes, if large enough it should damage the whole body
That is an interesting thought. If a creature is large enough their attacks should apply against the target as a whole.
I didn't include "Stomping" though mostly because even with creatures of equal sizes certain kinds of stomping can be an effective strategy (At least for people. Properly done a stomp can break bone). Crush is supposed to represent for the most part using the immense weight which would require a size/weight difference so there is less confusion. Falling backwards or frontwards onto someone of equal weight wouldn't hurt them too much, a cow, however, simply resting ontop of you could kill you.
Hmm I guess I could add another
-Move Through: Giants could kill, launch, or otherwise ignore other creatures simply by walking over or through them.
--Creatures of equal size could do something similar to this. Like a tackling charge, but they lack the ability to do so as casually as a truely immense creature. (Also I realise Toady has a version of this for Elephants and Carrivans)