I checked into the height limits on the Britax Roundabout rear-facing and found out that the older Britax seats still say rear facing until 30 in but now they've changed it and you can use it rear facing if your child is taller than 30 in if they still fit the other requirements (weight and ears not at the top of the seat). I'd call Britax for more information. I'd called them once about moving Lauren (my 3 year old) to a Star Riser/Comfy at 24 months since she'd outgrown her Roundabout and at that time the only bigger seat they sold was the Super-Elite which I knew would never fit in my car. They said it was fine as long as she was sitting properly (wasn't unbuckling her seat, etc).
I'm the one with the really large children who went nuts trying to find diapers for my middle dd that were big enough for her. She'll be 4 in 3 weeks and is 47 in tall and 46 lbs, and she was 36 in and 30 lbs at 12 months. It was hard since she hit 40 lbs and 40 in at 24 months. Not many WAHM's make XL's that really fit a 40 lb child with a really high rise to boot (she's much longer in the body). She just wore Toddler Cotton Kisses with the XXL Wee Bees side snap covers (she'd outgrown our favorite XL ME Airflows).
Lily is big as well but not quite so tall as Lauren was at this age. At 9 months, Lily was 26 lbs and 30 in and she wears a 2T, but I don't think she'll be 3 feet tall at 12 mo. But Lily wears XL diapers now. She was in larges at 4 months and on to XL diapers by 6 months. She also has the really high rise and a big belly
I only wish a big belly was as cute on me as it is on her LOL.