last year I started windsurfing and my boyfriend started with kitesurfing - so I think I can compare. (Sorry, my english is not the best).
I'm a "small and lightweight lady" myself and I started windsurfing at age of 40 (!). Of course it is hard, but I agree with Ellen that the way HOW you do it and the equipment is important. Also I think a good teacher who shows and explains the steps of a movement has great influence on sucess.
Once you know how to use a harness, I think kitesurfing and windsurfing need about the same physical strength.
When you learn kitesurfing you learn faster I think, windsurfing takes more time to learn. On the other hand a kite is much more dangerous (for the kiter and the others around). If a kite comes down uncontrolled it can hurt badly, also when the wind is too strong and pulls the kiter across the beach.
Hope that helps,