Guys don't believe women can grow big muscles because most women do not try to grow big muscles. But women CAN grow big muscles. Squats are a great way to get big thighs. You should see some of the college girls in the gym where I work with. Their thighs are awesome. As long as your waist is staying the same or getting smaller, if your thighs are getting bigger, it's muscle.
Female, 5'3", 49, Starting weight: 163lbs. Current weight: 135 (more or less).
Starting squat: 45lbs. Current squat: 160 x 2