|A pound of Muscle vs. a pound of Fat|
It doesn't weigh more, it is not heavier BUT muscle holds much more water than fat does. Therefore, you've technically gained water weight. Before you jump up to go buy diuretics, or push your water bottle away, water is a good thing. A GREAT THING.
Since you're exercising, you are probably eating a little more carbohydrates. This is also a good thing. Carbohydrates are your main source of energy, but that's a whole new topic for a whole new blog. Carbohydrates also hold more water though.
So now, when you hear people saying "muscle weighs more than fat....", you can say, "well, actually..." and they will be super impressed! Keep doing what you're doing. Focus less on the scale, more on the inches coming off, how your jeans fit, how you feel. Rather than focusing on a numerical goal of I want to weigh 130 pounds, focus on your body fat percentage, or have a goal of running a 5K. There's nothing wrong with hoping to lose 5, 10, 15 pounds but don't let it discourage you when it doesn't happen right away. Your body is still reaping the benefits of all your hard work.
Focus on being healthy and happy, less on being super model skinny.