Are you on a treadmill holding two 45lb plates?
Seriously though, the only way I can see you gaining muscle mass on your legs from running is:
A. If you are running with weights. And not just any weights...were talkin some big guns (like carrying VW bug on your back).
B. You have crazy resist on your treadmill + some crazy incline.
C. You had very little leg muscle to begin with. And now they are just developing to a normal state.
D. Supps? Creatine? Protien Levels up the yang? Carb frenzy?
How bout some measurements for a better perspective (thighs diameter, inseam, waist, calves diameter for fun).
And are these Fasted AM Cardio sessions?