growing out of stage 2 car seat but not 15kg

[hairtwiddler]

DD is 3 and a half and currently in Britax duo with isofix. Her head is poking out of the top and I was thinking of getting her a new one. She's not particularly tall or heavy though, so can't quite understand how this seat should go up to 4yrs!
She's 14.4kg - so not heavy enough for the next stage seats (from 15kg)
Stage 123 ones don't seem to perform well in safety tests.
Any advice. Better to keep squeezing her into the current one for the time being?

i put my ds was not 15kg when i put him in the next stage up, he was only about 13.8kg but he was far too tall for his old one. he hasn't a bit of fat on him although he can eat more than me. I took the thought that if something happened in his old one his head wasn't where it was supposed to be and therefore not giving him the protection that it was supposed to be.

For as long as her eyes are level with the back of the head rest or lower keep her in the duo.

Thanks both. Cargirl, yes it's only the top of her head that is sticking out of the top. She's not complaining so think no need to rush out and buy a new one until she has another growth spurt. Given the amount she ate today, that may well be soon!