My son was born big and grew very quickly in his first few months. But he hit 18 lbs around Sept 1st…

My son was born big and grew very quickly in his first few months. But he hit 18 lbs around Sept 1st and is still only about 18.2 at almost a month later. He's been exclusively breast fed and my supply is fine as far as volume goes - always plenty of milk. He's very active but shouldn't he have gained more weight than that in a month? He's coming up on six months old. Has anyone else experienced this? Cause for concern or normal? He's alert and happy but I'm still wondering if it's possible to have plenty of milk but maybe it's not nutritious enough? Any thoughts would be greatly appreciated. Thanks!