Baby's nappies leaking through

My son is 3 weeks old and was almost 11lb almost a week ago. The past week his pee has been leaking through his nappy. I change his nappy regularly and it's not always full when it happens. I thought it was just him getting to big for the newborn size so I brought him the next size up but it's still happening! Anyone know why!? It's not the brand of nappy either because I've tried changing that too