Well spotted Tim - there often seems to be territorial disputes between Iridomyrmex rufoniger (the smaller of the ants in the pictures) and Iridomyrmex purpureus (the large Meat Ant being pinned down). The Meat ants are big and can easily snip the smaller ants in two, but the weight of numbers and the rapidity with which the Iridomyrmex rufoniger can call up reinforcements often makes the them the winner in the end, though usually at great cost - the battle ground is often strewn with pieces of ant afterwards. It's a tough life for an ant! Cheers, Jon