How come my thighs getting fatter over the months whereas the size of my overall body remains?

Just the way you are. A person whose thighs are getting fatter faster than the rest of his body is, may be eating more calories than he needs (and so is gaining fat), has a natural tendency to deposit fat in the thighs (he's just made that way!), has a medication/metabolic/hormonal imbalance that is changing his fatty areas from what they used to be, or is gaining muscle (from weight training) deeper inside the thighs.