I personally would go the large depending on the sposie size you are using. And if you get the Mazalicious covers the snaps do cross over and make it fit. Or my tots bots fleece ones do up tight enough.

OK, when DS was 8kgs, I was using the toddler size huggie at night as it holds more, and the large cover covers it up as they come up higher on the tummy (higher rise). Now he is 10kgs and I am using a walker size at night, and still using the large cover. But as I said, I do use a bigger nappy to hold the pee...

HTH.