Im 13 about to be 14 and just barley 70 pounds. I don't want to become fat just gain like 5-10 pounds.im about 5 feet. I remember over the summer i would walk places every day all day and i went from a size 00 (in pants) to just a 0 slim. But when i went back to school and stopped walking i i went back down to a 0 and now i can't even fit that. So if you could give me some tips on how to gain a few pounds and tell me if its true that walking makes your legs bigger i would really appreciate it.