I always use the argument "terry nappies were the norm until very recently and no-one had difficulty learning to sit up when they had 4 terry squares fastened round their bum!" (my mum said i wee'd loads as a baby and had to put 4 on at night
Molly was quite late at sitting up, rolling over etc; she just didnt have much interest. The sposie nappy companies try to "brainwash" parents into buying super-slim nappies so they can "mover around faster" but AFAIK that is mainly just an advertsing ploy. Babies will learn how to do everything they need to do, regardless of their nappy size!
Also, when they learn to walk those big fluffy nappies will make a nice cushioned landing for their little bums when they fall over 15 times a day. I wouldn't bum-jump onto a tiled floor without a cushion tied to my bum anyway.
Not that I've ever done that...
And as the others have said, your LO is still young enough to not worry anyway.