Room occupancy based on a single motion sensor tends not to work in practice. Zone detection (multiple sensors) is good for turning lights on, but can still be tricky for triggering lights off.
The most accurate for room occupancy is probably a double trip beam where you can figure out which one tripped first and so distinguish coming from going.
Discussion of all of these here:
For a chair sensor, it depends on the exact use case. I disagree that pressure mats have to be bulky–they can be very thin and small, just a strip under the cushion. Coaster size, even. Often used in amateur robotics. Easy to make as well.
But certainly there are many use cases where an accelerometer or vibration sensor might work as well or better. 