DD2 was born 54cm 4.4kgs and I started using BBH with *medium* covers (the website sent me the wrong size but I kept them because they fit anyway) and haven't had any leakage problems. At least no more so than disposable huggies.
edit: I don't even know if the nappies I'm talking about ARE AOIs cos you still need covers for them! Either way they're OSFA and they fit my newborn.