Dd is 21 months and is beautifully rolly at 90cm tall. She is 98th%ile for height and around *correction* 80th%ile for weight so I'm not bothered. Even her ribs are in a funny place to accommodate that pot belly. Like every developmental stage, every child is different.
Eta-check her height and weight on the growth chat in the back of her red book and if you're concerned pop in and see your local mchn.