They buried the lede, and put half of it in parenthesis:
"In 1994 the average North American office worker had 90 square feet of space. By 2010 this was 75 square feet. (Executive management gained floor space over the same period, according to the International Facility Management Association.)"
There are not that many executive managers, and they get to decide what to do with floor space. It's like setting your own salary, which they also get to do.
Could this have something to do with economics as well? There are high returns to working in big concentrated cities. So if rents in city centres are expensive, but people still want to work specifically there, they will compromise instead by paying for less office space. How much more expensive would it be to give everyone an average of 90sqft (8.36m^2) of space?
Not that any of this is good, but to pin this undesirable outcome on a specific cause, I think one would need more precise evidence.
"In 1994 the average North American office worker had 90 square feet of space. By 2010 this was 75 square feet. (Executive management gained floor space over the same period, according to the International Facility Management Association.)"