Ok, I'm just typing this up, as a semi-rant, semi-advice thread.
Often, you hear the words, "Oh I don't want to be big." This is common among women, and among a ton of guys, generally quite often the overweight ones, or the guys that are ripped because of fast metabolisms despite whatever diets they've tried and failed on to get big.
Let me be the first to say, if you don't want to be big, don't worry, because you won't. Do you think that all the big bodybuilders, or guys with large amounts of muscle mass, just happened to pick up a few weights and accidentally blew up into some monster? Sorry, things don't work like that, especially in the world of natural lifters.
The amount of work you have to put in, slow metabolism or not, to achieve significant gains in muscle mass, is huge. So if you think you go to the gym and god forbid, put the pin in the stack one plate heavier, you are going to turn into godzilla, think again. You may get stronger, you may reap the rewards of lifting heavier weights in a progressive fashion can give you, but you will not all of a sudden get big.
Think of building a house, and all that goes into such an endeavour. This is the same as building your body. Lifting is a requirement yes, is lifting heavy, or extremely heavy a requirement? Not really. Continuous lifting, and fatigue, along with eating a diet with a decent macronutrient breakdown, of above maintenance calories, and plenty of sleep, and recovery time, are all key components of "getting big."
So, if you think that going to the gym and training 3 or 4 times a week, and adding an extra plate, or adding an extra exercise, or pushing yourself to the limit is going to accidentally cause you to turn green and gain 300lbs, don't, because it's not going to happen.
So train regularly with a routine suited to your strength, size or performance goals, and then allow your diet to facilitate whatever size goals, or energy levels you require for whatever you are trying to achieve.