My son was over 9 pounds at birth with chunky legs, we had a few BGOS that fit him right away, and he still fits into BGOS at 2.5 and 40 pounds. There was a point around 9 months where he was 30 pounds and was close to busting out of them, but he slimmed down a lot soon after.
We didn't use any OS diapers with DD as a newborn (I found out I much prefer sized) but she was bigger than DS and lots chunkier, so BGOS would have fit her right away, too, I'm sure. She started crawling and walking pretty early and slimmed down a ton, so I don't foresee her outgrowing them any time soon.
All that to say... We didn't use OS from birth to PTing, but we could have.