Don't worry about the growth rate, as long as it is eating he is good. I've had many motoros grow faster than other motoros--including rays from the same litter.--Like any other animal some grow faster than others and some grow larger than others, even of the same breed.
Your bigger ray may not be able to distinguish your smaller male as a male because it can't sense his hormones until it too matures. When I removed my breeding male motoro from the female and moved him into a tank with males only, for about a week he was biting and trying to mate with my male orbignyi and has now stopped.