My son is 3 yrs 4 months and weighs 19kg. He is in a Maxi Cosi Tobi. I note that the max weight is 19kg. Do I need to move him into a new seat now? He still fits in it fine, straps for fine, head is supported etc. He is 102cm so not a massive lad. How much does weight matter? What is the effect of being too heavy for a car seat?