What about Jean Paul Guilliame? He was pro and definitely natural, hard and tight looking. Not massive, but had big muscles.
Bill Davey and Mike O'Hearn are other natural guys.
You can get as big as your natural testerone levels will allow and maybe 10% more if you train hard and regularly. Once you hit that limit, you can get stronger but not much bigger. That's why as you get older and as Testosterone levels drop, you start to loss muscle mass. Same deal when you stop juicing, Test levels drop and so does the muscle mass.
Your hormones determine how much muscle your body can hold, your training determines if you can reach that limit or not.
Your can't keep getting forever.
Its much easier to get bigger from sacroplasmic growth - muscle goo. Typicaly what BB'es display. Big puffy muscles. Muscle growth from contractile proteins look diffferent, harder and denser. This is the muscle size atheletes normally display.