I've seen a few people mention going up a diaper size at night - LO seems to do okay with Pampers Baby Dy at night - the Swaddlers don't make it all night, and the Huggies 12- hour diapers don't either. If he wakes for a nighttime feeding, we change him then (or else he'll leak a bit) but otherwise, we're usually good to go. Last night, he didn't eat overnight, but did have oatmeal before bed - and for the first time, pooped overnight - and leaked. He's 19 pounds and in size three diapers - would it make sense to try size 4 for nighttime? FWIW, he often leaks when he poops in general - he only goes every couple of days, typically, but it's a LOT when he goes. He very rarely leaks with a wet diaper.
Re: diaper size question
I think going up a size would be a good idea for his size, but have you considered other brand types (ie. huggies little movers)?
Edit - felt a but like a huggies promoter here, but in all seriousness, we are huggies people. No leaks and snug fit.