The ROA is the annualized ratio of YourReward / YourStake. To optimize your ROA you would want to minimize the cost that you pay for these annual rewards. With an (almost) saturated pool that cost would be about 0.7% (i.e. pretty good)
Lets assume all goes well throughout the year and you make 5.88%, your friend who has delegated the same amount to a much smaller pool has a cost of 30%. Of course you could say that 5.88% vs. 4.12% is not a significant difference in annual return, especially not if the change of ADA value dwarfs both figures, but it is still 30% less rewards.
We could for example say: Delegator rewards are between 4-6% p.a. depending on pool performance, which is peanuts in comparison with how we expect the ADA value to change over the same time frame.
This would cover small and large pools and would at the same time be totally true and in line with what the reward formula predicts.