On the other hand T was 9lb born and 9lb7oz by 5 days, small ittis fit him straight away and I used them til he was almost 6 months....they still fit him but I was starting to prefer SIOs by then so I sold the small AIOs.
I've got an AIO and a SIO and both are fitting on Rory at the moment at 7lb ish. We had a leak when we tried them at 6lb7oz but they are fine now- and he has skinny legs. I guess it depends massively on the shape of your baby! They are lovely nappies! I can't wait until he fits the Itti tutto that I've bought!