I had the same problem with one of my boys and found the real easys great too - they are in the size m at 8 months and not growing out of them yet (babykind have them on offer at the moment)
I abandoned the Ittis, though I found the AIO better as it wasn't so bulky as the SIO around the leg but I had to boost it to make it work well which defeated the object of it.
With the WN if the leg fit is ok on the medium but the rise is low could you order some customs? They may well be willing to make some for you to fit your LO if you get in touch with her measurements/ the measurements you want. It could be a good excuse to buy some new ones