[manfredo]

That's a very bad comparison. The taller buildings are "less efficient" because they're way narrower than the short buildings. If you look at the picture only about 20% of the land is occupied in the tall buildings while 80% of the land is used in the 6 story buildings. If you raised the number of floors on the building on the left while maintaining the same footprint it'd fit a lot more units.

You either build wide or you build tall. If you can only have a 200x200 feet footprint, then building a 30 story building on that footprint is 5x more efficient than building a 6 story building on that footprint.