My experience is that after you are done growing, any changes in lean weight are due to glycogen depletion or loading. If I start working out with a small calorie surplus after dieting off excess fat, then my scale and skin fold caliper indicate I'm gaining a pound or two of lean weight a month. This continues for a few months until my glycogen stores are full and then I start gaining fat. When I try to diet off the new fat, all the lean gains disappear.
Many young men are able to gain some solid muscle during their first year of weight training. However, if this takes place during the late teens, then much of the gain would occurred even with out weight training. I gained 20 pounds between age 16 and 18. I also gained some height. I ate a lot and did some weight training but nothing compared to what I do now at 62. If you are weight training while you are still growing or are on steroids, then you need to eat a lot. If you're done growing and not on steroids then you should avoid surplus calories.