William looks like that too (but often has a big belly at bathtime after tea lol!), he's 24lb 3oz and 17.5 months old. Not sure how tall he is, I measured him a couple weeks ago and got 85cm and HV at clinic last week couldn't get any measurements as W wasn't happy boy there!
He's only just in trousers 12-18 months, but tops he has been for ages and vests he's wearing now are 18-24 months! His feet haven't grown in about 3 months so waiting for him to outgrow his next couple of pairs super quick! He's talking, walking from 11 months, knows 3 colours, 4 shapes, lots of animals and their noises so as far as I'm concerned, nothing wtong with him!
They all grow at different rates, I think as mum's we're worrying if there's nothing for us to worry about as much as when there is (if that even makes sense!).