I've got a Daiichi first 7 car seat for my five month old. It is a child sized seat but has an infant insert. the insert seems to leave a lot of space behind baby's back when sized to fit my above average height baby. The result is a fairly curved posture. Does anyone have advice regarding sizing the seat and insert?
If the space behind the back allows the baby to slouch (curved posture?) then the short strap between the legs is most likely not in the correct position. Adjust the strap according to the manual and if you cannot remedy the problem then the safest thing for the baby is to purchase a new seat that fits more snugly.