It was me that had that problem with OSFM. DD has chubby thighs. I have learnt that I just have to buy sized ones for her. The velcro OSFM didn't suit either.
As for the Ittis - we live in them! (Well, she does, I prefer knickers) DD is 12kg, still chubby in the thighs and can still wear her medium, though she is mostly in large because that's what I bought at the time when she was in the overlapping-sizes stage.
I never bought any Swaddlebees, but they are first on my list to try when #2 comes along -especially if he/she is similar shape to DD.