Leg muscles make you alot heavier?

sure, muscle that has been somewhat "inactive" gets "active" and will hold more water and glycogen- so thus the extra weight - 75% of strength and "new" muscle come from neurological response to exercise, so if that muscale is now healthier and more active it will require more nutirents, water and glyco - makes perfect sense

I think you're a little off. While biking and running will add strength and some size to your legs, unless you were bed ridden for a long period of time, your 12 lb. gain didn't come from biking and running. Have you been eating a lot of pasta and pizza lately? Seriously, doing heavy squats and lunges for years will add the size your suggesting is the reason for your 12 lb. increase.

Well I started working out more this week more water/food had pasta yesterday never any pizza, etc stuff like that