What is a reasonable amount of weight gain per week after starting weight training program if diet remains same? muscle weighs more than fat....

Weighs the same: A pound of fat weighs the same as a pound of muscle. Fat is less dense, takes up more space. As you get leaner, clothes tend to get looser, even at the same weight. Add muscle mass to that and clothes may get snug, e.g. Broader shoulders, bigger biceps. Want weight gain? Will need to increase calories in, and/or decrease calories out.