dunno if its true, but I've heard that it takes a while for a baby ray to reach 15 cm, but beyond that, they should grow fairly faster till they reach around 25cm-30cm, then it slows down. In fact 2 of mine are at size which is smaller than yours, I havent seen much growth eventhough they eat alot daily.
That's a pretty motoro. What are you feeding? In my experience motoro females grow rapidly ~12" the first year and then growth will begin to slow. My female is 2.5 yrs old and 17" across or so. Gravel isnt ideal for small rays as they can't bury themselves very easily. As long as she's eating well and your water is good she will grow for you.