I don’t buy this. It seems they’ve tested one potential explanation only, GDP. What about the size of the city per se or mean walking distant between objects? A

I don’t buy this. It seems they’ve tested one potential explanation only, GDP. What about the size of the city per se or mean walking distant between objects? As example: Berlin is a city which is not compact and quite spread out. What if people just need to walk quicker there to make it to work on time? I am not saying this is true. But I want to make a point that you could come up with other things to test easily.