How do you thin out muscular thighs?

I have moved into my new home 6 weeks ago. It is a three story town home and I am constantly up and down the stairs. I have noticed that my thighs have gotten muscular and thicker. All of my pants are now tight. Although I appreciate that I also getting toner everywhere else, I do not like having thick thighs. My thighs have always been easiest part on my body to get muscular, so I never would do exercises that would bulk them up. Unfortunately, I can’t simply stop going up and down the stairs. Things always need to get done.
I’m hoping someone in fluther has tips (other than stretching half my day away) on how I can counter the effects the stairs are having on shaping my thighs.