I agree with IamA.Kat
All mine were in the 90th percentiles so we ran into the problem you are describing.
With our last they had come out with the convertible seats and they went from infant to child booster. We still had a click and go infant bucket seat for convenience to use with strollers but by this point, she could sit in the stroller without needing to be in a car seat.
It was great. You just keep adjusting as they go. I didn't it too big either. Easy to get in and out of cars without too much fuss.
This is an example of one (ours was similar)
http://www.toysrus.com/buy/booster-car-seats/graco-extend...