Okay I am 6'1" 180 Ilbs.I am an 18 year old male, I am weightlifting to gain more muscle mass, how many calories a day should I be eating to bulk up more? and why is it that some bulky people look slim with clothes on but under has lots of muscle mass? I was just curious about this!

Well, I wish I could tell you that proper diet is my area of expertise, but, I'm actually pretty bad at it. However, I can maybe help you out a little. I am 6'ft and a 195 lbs. I used to be around 160. Very thin. Very scrawny. I'm much bulkier now, but I've also put on a few extra lbs of fat. So, Basically, if you are just trying to gain muscle, you have to a eat a ton. And LOTS of portein (I.e. meat). If you just wanna gain mass, you can't really eat too much. BUT you have to work. You have to lift, and you have to lift hard. If you're just going for size, lift as much weight as you can. Get a split routine. Like my weekly routine is: Chest/back one day, rest the next day, bi's/tri's the next day, legs the next day, two days rest, chest/back, repeat. I also do abs and cardio, but I just throw those in whenever I feel like it. So, to put it as simply as possible; the two steps to gaining muscle mass are: 1.) Eat lots 2.) Lift very heavy amounts.