Yes dogs can die from getting pregnant from a dog much larger and the puppies being too big. This is a legitimate fear. Please keep them separated and if possible get them spayed/neutered asap. Many places offer free or discount spay/neuter, especially for pit bulls. The spay/neuter clinic near me will spay/neuter pit bulls for free. Also to be able to deliver the puppies at all she would very likely need a c-section they won't fit through the birth canal, this is an expensive procedure but if she is pregnant and can't deliver the puppies it's either that or she will die.