Diaper Question

So my son is 8 months old and has been wearing a size 4 diaper for about 2 months now. He technically should be in a size 3 because of his weight but he kept peeing/pooping out of the diaper so I tried a size 4 and it stopped and we haven't had any problems until now. Lately he's been peeing/pooping out of his diaper again and I don't know what to do this time. I change him often so it's not that he's over packed. I feel like a size 5 is way too big for him. He weighs 18 pounds. Has this happened to anyone? What did you do? Or if anyone has any ideas please let me know

Thanks in advance!!!