Yes, those canteen cups take up almost no space.

Though I prefer stainless steel (mug, pot, bowl) since the rim cools quickly and you can drink from them directly. It doesn't need to be from an outdoor retailer either -- I have sometimes taken a small, inexpensive kitchen mixing bowl in a daypack because my spare clothing conforms around it perfectly.

Edit: It's important to choose something that's safe for contact with food. Even an uncoated tin can from coffee is useful.