I have seen the WTS close off some sections for various reasons, not so spread out, "staff" to clean or for attendants, less required; optic reasons perhaps for jws pictures; views from that location not so good. I can't imagine that the WTS overestimates attendance due to cost concerns, renting/leasing a facility that is too large would undermine the bottom line; attendance is notoriously poor on weekdays and swell on the weekend and/or the day of the drama (sic).
When the circuit assembly hall I attended saw dropping attendance, they readjusted from A,B,C circuit assemblies to only A,B segments. Once again though, seats were hard to find. Local halls do the same, remodeling and taking seats out (too many from the pre-1975 days).