When they are little the bag can come up and cover their nose/mouth if it's too big. That's why there is specific sizing.

I've had 3 babes and all preferred different things. DD1 a tight stiff swaddle, DS a looser swaddle (so we used a velcro swaddle) and DD2 a bag.
I've always started with a swaddle and seen how we went from there.

DD2s bag is made of bamboo and goes from 3-24months (from Natural Kids). It has press studs in the armscythes for smaller babies so that the bag can't ride up.