A larger rear rotor will also lower your brake temperatures, thus preventing the brakes from overheating/fading/etc as easily. Depending on how fast/heavy you are, and what type of riding you enjoy, this may be a very desirable outcome.Because when you brake your weight shifts forward so your front brake does 70% of the work. It's the same reason cars, motorcycles etc have larger front discs. Putting a larger disc on the back will do nothing than encourage you to lock up the rear wheel.
200mm are just so much better and i cant imagine the weight is perceptible to anyone- so why is every bike sold with a 180mm rotor on the rear?
This may be your experience and this is cool.The reason not to run large rotors is they rub when the frame flexes.
They also are not really needed. Rear brakes are used a lot but they are not used particularly hard. When is the last time you thought, "I just don't have enough rear brake". Unless you are riding a fresh super tacky DH tire on a DH bike and are really good at finding traction it is unlikely that you will ever exceed the capacity of a 180mm or even 160mm rotor on the rear.