The legs hide a lot of fat due to the large muscles in them. Most people think their legs are lean, but they really aren't. The larger the muscle group, the more fat can be present without looking the part. Diameter * 3.14 - Remember that. Average legs are 22-24" circumference, arms are probably 13-14". A world of difference when it comes to how much fat each can hold while looking lean.
Additionally, because the legs and glutes are the largest muscle group (besides the back), they hold a lot more glycogen. If you never really worked out before both upper and lower body had small glycogen stores. Once you started working out, they both enlarged. The legs hold a lot more glycogen because they are so much larger. You would be amazed at how much the legs shrink when you drop body fat. They don't "look" like they shrink as much, but they actually do. Even at approximately 10% BF, my thighs are surrounded by approximately 5mm (10mm diameter) of fat. That means, if I were to lose all that fat, my legs would shrink 1.23inches in circumference. Obviously you can't lose all that fat or you would be dead... However, when I was heavier at 20% BF, my legs still appeared lean, but they were around 22mm. So they shrunk approximately 1.5 inches. It is important to note that even at 22mm thigh measurement, my legs appeared "lean", yet they were far from it in reality. Then again, everyone has a different idea of what lean is, because it is somewhat subjective.
TLDR - Provided you are doing full body work outs properly, it would be impossible for your legs to grow, but your upper body to not grow. Whatever you see here is an illusion and is likely obscured by body fat. Again, assuming you are sufficiently stimulating growth.