I am over 200 also, and I think about this all the time.
Here are some things that I feel pretty strongly about.
Does your saddle fit you, as in really fit you? Not that you are cramming yourself into a saddle, but one that you have given the same thought and rules of saddle fitting into, just as you did when you were thinner.
That means, if you should have a hand's breadth behind your rear at the cantle? It is there, no matter how large your hiney is. And if you have belly fat that hangs down in an apron? Is it interfering with your hands? Without a correct fitting saddle? You are doing damage to not only your saddle, but your horse too more than likely.
Can you post, if you do that, without hauling on the reins? Are you able to ride at any gait and still be in balance? Have you taken into consideration the differences weight will cause both for you and your horse?
Pay close attention to your stirrups too, are they at correct length? When we put on fat, it usually settles in thigh area and thus will make the stirrups fall different, and make it harder to get our legs in correct position.
Carefully assess with aid of video camera if possible your riding, and watch for signs that your horse is experiencing discomfort. Head tossing, ears back, rooting for bit, tail swishing, or a generally unhappy look to horse? Not a happy camper.
Being heavy puts added strain on both us, horse and tack, so you must be more diligent than others about checking out if horse is sore, if there are leg problems or if we need to switch horses, tack or disciplines.
Great courage to take time to worry about this openly, I commend you for this.
Horses make me a better person.