I don't have the physique of Jennifer Aniston, but I haven't gotten "big and bulky" using heavy weights for lower body (when working out in a gym and using 150# or so for squats, and heavy leg presses etc.
I think it depends a lot on your body type. Someone who has a lot of mesomorph (builds muscle easily) would probably have a tendency to 'bulk' more easily, especially if they also have some endomorph (tends towards more fat)mixed in. Someone who is more of an ectomorph (hardgainer, who has a hard time putting on muscle) wouldn't bulk. I also think some people have more of a tendency to build muscle density over muscle size.