That's an interesting point. In Rutherford's day, when the whole org was being weaponised into a door to door sales business, all the paperwork would have been literally on paper and would have needed an army of clerks to administer it. Hence the large Bethel presence and the farms to support them.
I imagine there has been resistance to downsizing by the old guard because they would remember the glory days of a huge Bethel workforce which is indeed, as you say, just not warranted these days. They would probably rather be surrounded by a large, bloated group of Yes Men, than a small but efficient one.
Do the farms still exist in any form?
I'd say that your thoughts on the matter are sound. If they thoroughly analysed where they want to be and the manpower requirement for that goal, a much smaller administrative staff would probably suffice. But the GB probably still hankers for the good old days...