###You should check the rules on Dragging. While you could still grapple such a creature, you would also have the option of dragging it. Your choice depending on which rule you want to use. I would use dragging myself.
As per the PHB p176:
###Push, Drag, or Lift. You can push, drag, or lift a weight in pounds up to twice your carrying capacity (or 30 times your Strength score). While pushing or dragging weight in excess o f your carrying capacity, your speed drops to 5 feet.
emphasis added to the word weight to show that you can drag creatures (who are not resisting) or objects