Actually I have a turtle just like that.
About 3 years ago there was a pet store near my house that sold baby red eared sliders. My cousin bought 1 of them, I bought like 4. Well only my cousin's survived, and he gave up on it and gave it to me.
I had that turtle for 3 years, all the while it was still the size of a quarter, I figured it was a dwarf as well. During the summer of last year I traded it to me friend, he gave it back in December, it had doubled it's size.
All he fed it was worms, I was only feeding it turtle food. I think yours will grow, keep the water warm and feed it everyday, turtle food, chicken, turkey, fish, worms, I can almost bet on it growing by the time summer comes around.
Michael

Michael's Place