The problem with my question is that 'str' and 'toughness' are stats
What I have noticed is that tougher models are usually bigger. A Dreadnought is bigger than a Terminator which is bigger than a normal Marine.
But skill/magic/technology should make a soldier relatively tougher for its size. The Tau weaponry tends to be better than Imperial weaponry.
So, which model is toughest relative to its size. Or, better way of asking, what's the 'toughest' model that relatively small?
edit: shoot. Meant to be in 40k General Discussion.