This statement is unfortunately not true at all.
Assuming 0% margin for better comparison, you have the same (fix) cost for all polls. For a small pool that makes 1 block p.e. the cost to the delegator will be > 30%. For a large pool, the same cost (i.e. 340 ADA) is shared among many blocks. A more detailed explanation is here.
You can also use this calculator to explore owner reward vs. delegator reward for various size pools. You will notice how delegator reward changes quite significantly depending on pool size.