Uppababy Mesa outgrown already

Brit

Anyone find the Mesa small? I’m a FTM and my 7mo guys head is 1” from the top of the highest setting already. It says good to 35lbs or 30 something inches but he’s only 17lbs and 26in and his head is already at the top. I’m confused. Why is he outgrowing it so quickly? He’s not even large on the growth curve. Do car seats “run small” like clothes? 😂