Assuming your interpretation were correct, the following would likewise be completely legal:
Polymorph into a Stone Giant (or any other of an assorted list of Large-sized butt-lickers) and then cast righteous might to become a Huge death-dealer.
Since, by your reading, I am not "gaining size" by polymorphing into a giant (because that is its "natural" size), I can further increase my size as such using any of the size altering spells (like righteous might). I don't think that's going to fly.
Polymorph into a Stone Giant (or any other of an assorted list of Large-sized butt-lickers) and then cast righteous might to become a Huge death-dealer.
Since, by your reading, I am not "gaining size" by polymorphing into a giant (because that is its "natural" size), I can further increase my size as such using any of the size altering spells (like righteous might). I don't think that's going to fly.