Why only height and not weight?

In the UN r129 (‘i-Size’) regulations, the classification of car seats has changed from kilograms to centimetres, since parents often know more accurately how tall their child is. This makes it easier to choose the right (and safest) seat. The iZi Flex FIX i-Size is designed to secure a child in a great way through the entire height range, independent from the ISOfix related weight limitation.