The efficiency issue isn't building the buildings, it's how much farming you get for your use of space. Generally speaking, big buildings are for housing people or giving them places to do work. So if you're going to justify the cost of building a big building just for farming, it has to beat out just using that same space to give people a place to work or sleep. Not saying it can't, just that it's not whether we can build it, but what alternative uses we could put to it that should be the main point of comparison.