Even for the same amount of living space, apartments are way more efficient. A typical apartment unit is surrounded by other units up/down/left/right, so only two sides are exposed to outside air. A single house is exposed on five sides.
Six. The most heat escapes through the roof, but thermal loss through the floor is generally about 10-15% of the total.
Same goes for the slab/foundations (which can also have substantial thermal transfer in many contexts).