When something is really big and holds a lot it is capacious, like a capacious purse that is so big, people mistake it for a piece of luggage.
Have you ever seen a Fourth of July hot dog eating contest? As you watch people wolf down 60 or more hot dogs in a matter of minutes, you must be thinking, "Where do they put all that food?" Well, it helps to have a capacious stomach. The suffix -ous adds "full of" to capacity; capacious is literally "full of capacity." If something is capacious, it has plenty of extra room.