AND, even if you have a tiny kid in a larger seat, it doesn't mean you HAVE to keep using it till it's outgrown. It just gives you the coverage in case they grow, or if you simply want to use it longer. My kids were in Regents for a couple years, I didn't make them sit in them till they were 10 when they actually got too tall, but we loved them when they were 5-8. A larger convertible is maybe only a couple bucks more (I mean, if that... a True Fit is cheaper than a Roundabout...)than a smaller convertible, and you need one anyway, so you aren't going to be out extra money for an extra seat you only want to use for a couple years...
Anyway... My petite DD 1 was happy to ride in her Marathon up through kindergarten (she rearfaced till age 4.5, then fit in it forward facing till age 6 and about 40 pounds when she got too tall for it..she rode in the Radian till she was 7 after that happily).