N.,
I had the same situation. When my son was about 12 months old, he kept leaking through his diapers. We did go up to the size 5's even though he was only about 20 pounds. He has no waste and it very thin and we had to overlap the tape in the front for them to fit, but at least they worked. He is still on the 5's and he's only 18 months old and about 23 pounds. He drinks a lot of milk, juice, and water during the day and loves his milk before going to bed and we didn't want to change that because we think it is important for him to have all of that. So going up in size was our only option. He still sometimes will leak during the night, like after our last trip he slept for 12 hours when we returned. We expected that. I've even changed him in the middle of the night just to stop this from happening.
Good luck.
R.