Sounds to me like you're just adding some of muscle. Now, I could be just rationalizing my "fat" days, but I sometimes feel like my muscles, especially my leg muscles, literally swell after a hard/new workout. For instance, when I started doing some weight training this fall, I could swear that my thigh muscles were swollen after the first couple of workouts, almost like they were retaining water. I imagine there's a scientific explanation for it, e.g., increased bloodflow, but I don't know that for sure. In any event, they always return to normal after a few days.
I, personally, lost some inches in my upper thighs, hips and butt when I started riding. I also lost about 6 pounds, which I managed to keep off. My mid- to lower thighs, however, have gotten bigger, especially from spinning over the winter.
I agree with Lisa, though. The goal (hopefully) in exercising is to get more fit/stronger. If that means bigger thighs and/or booty, I'd encourage you to embrace that. That said, remember to watch your calories intake, too. You need to fuel your increased activity, but don't overfuel it.
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher