I had a 10lber this time and can't use the small BBH petite I bought to try. Like Maz said, they are extremely small. It's got a very absorbent snap-in part though - too much for a newborn pooing all the time IMO. I did use it once on day 2 or something with just an IB mini booster laid in instead, but it was still a tight fit.
What am I trying to say?? lol Size wise it's probably just right for what you want but if you don't use the parts it comes with, I guess you might not find it worth it because you'd use it more like a cover...ramble, ramble...