I'd stop worrying about your current pants and pay attention solely to the progress you're making on your body. Maybe all your pants are "slim fit" - made to fit women without a lot of muscle in their legs. You're building nice, strong, shapely legs. Perhaps you could buy some skirts & show those suckers off!
So what if you are building muscle under a little fat. Eventually that fat will burn off. And, it sounds like you've bought pants for one body type - lean. It's ok to buy pants for your new physique - shapely and muscular!