To leave the more thorough explanations to the docs, taller, leaner types typically do not withstand G's as well as shorter, stockier builds, a result of the blood having less distance to travel and more mass with which to constrict its movement.
Feel free to expound / correct.
Yup, what he (and others) said. Pulling Gs puts a pressure overload on the heart; train accordingly with weights, tossing in some cardio to increase your ability to tolerate repeated bouts.