Walking steep hills makes my jeans tighter?

I spend 2 hours every day walking. My route includes brisk walking on flat ground, climbing steep hills, jogging, and sometimes sprinting intervals. I also do one hour of vigorous yoga every day. On most days I consume around 1500 calories per day. I lost weight and have now seem to hit a plateau. In fact my jeans are starting to fit differently, tighter in the thighs and butt. What is going on? Am I building muscle? Are my legs and butt getting bigger from walking? Is this possible?
 
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