I tried fasting in the winter once and it was harder because I craved hot drinks like teas and coffee.
It does lower your body temperature, but I'm big too, and I think if your big (obese) in the summer, it still feels really hot and you still sweat a lot as opposed to someone with less padding fasting.
I'd say just drink water if your thirsty, you are sweating more, so your losing more water and drink more.