That is true, it's not the best way to compare heights but I believe in this example it's appropriate because of how close the buildings are. The smaller of the two unknown buidings is about 550 feet away from the Weston building. The larger of the two doesn't seem to be too far from the smaller and it seems to be 150-220 feet taller than the Weston, which would put it at about 550-700 feet. As for the Weston and BoA building, you can tell there is a gap between the heights. The distance between the two is 550 feet as well.
