We had a problem with the rear-facing seat, IIRC, in that my daughter at those ages was too tall but did not weigh enough. So her legs were literally sticking into the upholstery of the seat and as she grew taller, she could not extend her legs at all. That did not seem "safer" to us, so we turned her around a bit early weight-wise so that she could move her feet around. She got a foot stuck once when she was kicking away, and that was what prompted us to turn her around (I don't remember the age, though, now that she's 14!).
She definitely took forever to grow out of things, and was wearing 2-Ts to pre-K at the age of 4. Now she is a bit shorter than most of her friends and still slender, even though she wasn't all that small when she was born. I grew 5 inches during my three high school years, though, so I keep telling her not to worry, she WILL grow!