Tiny has it correct in that the size of your thigh will turn your knee out and also you toe.
I am not one to help with loosing weight in particular body areas but, one thing you can do to hep with the placement of your thigh is, when you first get on and several times during the ride, is to place your hand, from behind, under your thigh and pull all the flubber ( sorry, muscle) towards the back. This will place your knee flat, toes will point forward more easily and the inside of your lower leg against the horse's side.
It will feel strange to start and you need to be aware when it all slips forward and pull it back again. It doesn't take long before it becomes natural and remains where you want it.