Not always!! Ive seen cocks that were like 2" when soft but grew to 7" when hard. And if a cock is 5" soft, it may only grow to 7" or 8" too. Of course it can be bigger. Not all cocks are the same. Just because one soft cock grows to 3X+ the size when hard doesnt mean they all will.
Hence the saying "Im a grower not a shower" LOL