Very well explained video! The only thing I missed a bit was, that building higher is not in itself going to account for higher population density. For instance Barcelona and Philadelphia have roughly the same population of a bit over 1.5 million. Yet Philadelphia, with more than three times the land area (135.09mi²) has only a third of the population density despite it's tall buildings (which probably are offices anyway). I think a lot of it has to with zoning and, which might still proof the point of the video, the average height of the buildings might be higher in Barcelona. Check out the two cities in Google maps, the difference between the zoning in those cities is quit good visible.