The thing with the thumb is indeed a matter of training. But it does bring the problem that people who are used to a different device might not enjoy the experience, and decide to use another device.
The male vs. female perspective doesn't come only from the size of the thumb, but how we are used to handle objects. There is this whole "be delicate" thing that my mother tried to teach me, that I can use now with touch screens.
There is also the aspect of the length of the fingers. Ideally you should be able to reach both left, right and center of the screen with your dominant hand to be able to easily use the reader one-handed. And for this, narrower margins help, especially for women.