Clix in a small for us too, Bea has had it since she was about 13 weeks, she still wears it now and she weighs just over 11 kilo's, strong , comfortable with good reviews, I always thread the seat belt through the bit where you "clunk click" it as it gives her more room to move and lay down.
Bea loves the car and my car is not designed to keep things in the boot safe, hence back seat driver !