Re: Bigger kids in harnessed seats..
The most recent pic I have is 6 months old and the wrong car seat. Here's a pic from that warm spell we had last march - DD1 would have been 4.5, ~43" and about ~43lbs and she's FF harnessed in her True Fit.
She's a little bigger than that, will be 5 next month and currently rides in a Nautilus. If she continues to mellow out as she has been I'll start booster training her in the spring. She's a pretty laid back kid.