It's a great painting and I don't think the body is out of proportion to the breasts - but that will be because personally my breasts are larger than my head too. I am also very high waisted so don't think the belly button is out either.
Or maybe I'm just sensitive because I would look like that nude.
To me it looks terribly realistic and I say terribly because ageing is hard for society to embrace and I can see from Sewells description his reaction us quite visceral - it doesn't evoke that in me but I can see why it might do in an elderly, not ageing prettily either, man.