If she isn't growing super fast swaddlebees nb's might work as they go to 18lbs.
I had that issue with my scrawny dd (but at 11 weeks she was maybe 8-9lbs) and fuzzibunz xs were what fit good with no bulk.
Next up for OS was grovia. Both the AIO and cover worked.
But bulk only really bothered her at night and naptime when laying down.