That's correct so far. Now think further: a single, larger rocket has the advantage of leaving away the tank walls that would be in the inside now. Less mass ne

That's correct so far. Now think further: a single, larger rocket has the advantage of leaving away the tank walls that would be in the inside now. Less mass needed for the tank walls (the outer walls need to become stronger though), more available for payload, extra fuel and oxidizer or extra stuff like things needed for reusability.