It doesn't really matter. But if a 200lb person is riding a pony then that will jack up their back
The lighter the better I would think. Generally I feel uncomfortable watching anyone over 250lbs riding horses, male or female.
Generally in Australian competition you can only weigh 15% of the horses body weight. If you are any more you put too much strain on the horses back which is very sensitive and can be easily injured. Hope this helps!!
There is generally a 20% rule of thumb. 20% of your horses body weight should roughly equal youre own or less
It depends what breed of horse you ride