Whichever country (given income equality and ignoring cultural disposition to consumption patterns) a human would need approx' same amount of goods, whether grocery, clothing or other such, and those items would occupy exact same space in all countries.
Not at all. Consider a toy store in Manhattan(or Tokyo) vs. one in Des Moines. The former would be naturally compact due to the expensive real estate while the latter could be far more expansive. By extension country-wide retail square footage can be expected to vary by density (in addition to other cultural factors).
I still remember visiting one of the Fifth Avenue ones as a kid. Because yes, apparently you can build something as big as a department store, but all toys. :)
I have visited it once I think, but that was way before having kids. Feels like it would be a two-day exploration marathon now with our two boys. Hopefully travel will make sense at some point again.
Store layout and the space allotted for each product differs between places, in other words. I'm going to assume things like shelf height does as well. When these things are in the back room, they don't take up space in the same way as the sales floor. Also, it is quite possible that many retailers simply do not have the same sort of selection as they do elsewhere: If you sell fewer things, you don't need so much space.
This would have the side effect of including varying levels of foot traffic (in revenue) and real estate price (in cost), which seem the two biggest differentiators.
This allows them to have smaller back rooms, fewer warehouse deliveries, and to restock only once a day.
Note that compared is key above. Real estate and labor are both more expensive in dense cities, but comparatively real estate is more important to optimize and so you pay the higher labor. Even though labor is cheaper in suburbs you sill optimize that because real estate is so much cheaper.