It makes sense if you think about it. I live in a relatively dense neighbourhood, and if you consider the infrastructure my building and the building across from me share, two sidewalks and a street, it's already almost as much area as the footprint of our buildings. Add to that large buildings that often have huge paved areas around them, and highways which are extremely wide and stretch for hundreds of kilometers, it's easy to see that there will be more infrastructure than buildings.

With regards to the grass, even though I live in the city I already have as much garden ("urban grass") as I have building footprint. I wonder if they include other garden vegetation as "urban grass" as well, but maybe it's not significant in comparison?