I used the Grobag 0-6 month ones on dd from about 8 lbs (she wasn't 10lbs till about 3 months). I sewed the arm holes up about half-way up so she couldn't bring her arms inside. I think the "from 10lbs" thing is because they could slip down inside, if they pulled their arms in and their head is still quite small - dd certainly could have, hence the sewing.
With ds, who was even smaller, I bought the tiny newborn size.
Except I've just now looked on the bumpto3 website and they've changed the sizes. The first size is called newborn and is from 7lbs, up to 6 months.