If they're already bulky, try not to work the larger muscles, if you do try to go for a more aerobic exercise. In the end though, you can't fight genetics. If your legs are bulky and muscular, just enjoy them. FWIW, speed walking helps slim mine down, or at least they look leaner.
(I knew a girl who was pear shaped and had lipsuction to make her legs thinner and oddly, her legs went back to what they were? She was very thin and her calves and thighs were very thick. She was a runner and pretty dang healthy,too... I guess that is definite proof... you can't fight genetics? )
I love walking! I walk 20 minutes down to the city centrum every morning when it's school to catch the "second" bus there, instead of taking both the busses.
I also walk home from nights out, because it's that late that no busses goes anymore, and I can't afford to pay for a cab (though this makes my mom stress out, she's afraid something will happen with me, as I mostly walk alone then...).
I always trip up stairs, I never take elevators. My sis teached me to trip on my toes up the stairs, because that would exercise the muscles at the backside of my leg.
And then I walk a lot just to walk, because I like it, and I always get to think a lot. It clears my head in the same time my body feels good.
Though I'm a bit worried that walking is the reason why my legs are a bit bulky and my muscles too big. I hate that.
Sacrifice is giving up something good for something better.
I don't have a car, so I pretty much walk everywhere. I would estimate about 5 miles a day, 5 days a week or so, since the closest bus to my house is 2 miles away. I love it since I don't really work out, it keeps me thin and I can still eat.