On all-glass web site, the dimensions listed for the 72g are 48x18x22. First, those are all outside dimensions. Second, 48x18x22/231 = 72 gallons of water. I would surmise, that the tank is ~48" long, is probably not 18" deep in the corners. It's probably the bow portion. That would make the over all footprint 48x18. I have no idea what the corner dimensions would be at that point, but I have seen links to calculators that may help (which I don't have - sorry).
Of course, a 72 gallon tank does not hold 72 gallons (and never has), so that further supports my conclusion. HTH