I have to run to class soon, but my short answer would be a change in attitudes towards construction since 1931, as well as the development of new techniques. The parking ramp will not be all steel girders and glass, but mainly made of concrete, as I understand it. You also had a great many more workers in 1931 in NYC who could be set to some goal, and with a building as high as the Empire State Building, you could follow that steelwork with the rest of the work as the building got higher, something that it seems you cannot do with a small parking structure. The trend now seems to be to stretch a project out with fewer workers than to get it done quickly withmore workers.
EDIT: Fixed my mistake