I use to have my seat too low when I was in high school. The quad muscle closest to the knee, on the lower inner thigh got very big; it was mostly just that one. Now that my seat is higher, the muscle closest to the knee is almost the same size as before, but now all of my outer thigh muscles have bulked up. My guess is you use a lot more of your quads when the seat is higher and those are your biggest muscles.

Don't get too hung up on not being able to put a foot down and remain seated when you come to a stop. You're not really suppose to. When you brake, the bike stops before you do, so if you stand up, you will naturally move forward off the seat. When your wheels come to a dead stop, they don't move, so if you hold the brake, you can stand on the pedals, take a foot off while standing on the other one and put it on the ground. The easiest way to mount is to have a pedal in an upward forward position, stand on it as if you were pedaling standing up while using it as a stepping block to sit on the saddle.