The thing is it doesn't have to quadruple in order to justify the extra real-estate cost because your monthly salary is (probably) much higher than the monthly rent on your desk space.
If you're paid $120k (10k/month) and rent is $5/sq-ft per month [1]; going from 36 sq-ft of space for your desk (small) to 144 sq-ft of space (4x as much) will cost $540/month. Meanwhile even a 10% productivity decrease would cost ~$1k per month.
Edit: Thinking more about it, the productivity loss is even worse than above. You might be paid $120k/year, but the company expects to extract some multiple of this number in value from the work you do (I've heard target numbers like $500k/year) - so a 10% decrease in productivity is more like $4k/month in lost value.
[1] https://www.quora.com/How-much-is-typical-office-space-per-s...
Additionally, high overhead increases risk. Maximum productivity is only important as long as you have more work than there are employees to do it. What if there's a downturn (as in 2008)? Now you're locked into an expensive lease and no longer have the revenue to support it. The company with lower overhead can hold out longer until things turn around. Employees can be laid off. Getting out of a lease or selling a building are harder.
For example, a company I used to work at sold its building and rented (leased?) only three floors and the basement from the new owners. This injected cash into the company. I don't know what the slightly smaller cubicles cost the company in terms of employee productivity. How do we quantify such things?
The next place I worked (steel industry) at had my cubicle which was at least four times as big as that place before but I'm confident my productivity didn't go up four times because of it.
I'll agree with gp though. It is reprehensible to defend open floor plans as anything other than a cost cutting measure.