I see a lot of people struggling to gain mass (I was once one of them). This is mostly due to the misconception that mass building occurs in the gym. Actually none of the mass building takes place in the gym, in fact the exact opposite. When you perform strength workouts, you are essentially tearing down muscle fibres in order to promote the growth of new fibres in their place. The repair process usually results in more fibres than were there originally and a net result of an increase in muscle mass. However, in order for this process to work it has to be precisely fueled. Over fueling can result in undesired baggage weight, while under fueling results in no gains in mass. More often than not, people take the under fueling approach through fear of gaining unwanted weight, thus struggling with gaining muscle mass.
Over the past year I have experimented with the "Eating big" analogy to find where my limits lie. First of all I started with John Berardi, a professor in sports nutrition. In his book "Scrawny to Brawny" he describes his struggle as an Ectomorph to gain muscle mass. But to his realisation he too found himself eating way too little when what he thought was eating big. In order to calculate the exact calories your body requires for optimal muscle gains, get his book or read the following article on calculating your RMR (Resting Metabolic Rate) and ERAT (Exercise-Related Activity Thermogenesis):
The Cunningham Equation - An accurate way to predict calories
If you are a similar height and build to me (5'8", 85kg, 15% BF) you will discover you need something in the region of 4200 calories on training days and 3600 calories on rest days in order to gain mass. This is a lot of food and just to give you an insight, heres a typical workout day meal plan I follow:
Breakfast: Jordans Nut & Seed, Greek yogurt (620 cals)
Snack: 2 bananas, Half a tablespoon of peanut butter (360 cals)
Pre-Workout Meal: Heinz lentil soup (166 cals)
Post-Workout Meal: Spinach, Broccoli, Carrots, Runner beans, Chicken Thighs, Cottage Cheese, 2 Omega 3 Eggs (1030 cals).
Snack: 2 bananas (210 cals)
Evening Meal: Can be anything, but I usually go for meat and veg combo, similar to lunch but more calories (1500 cals).
Pre-Bed Snack: Carbs release melatonin which helps me sleep and gives me a boost in the morning for my 6:30 workout before breakfast (Jordans again but with semi-skimmed milk instead of yogurt) (300 cals)
Thats a typical day and in total all that adds up to 4186 calories. So are you eating enough?
For help working out the calories you are eating, checkout the Calorie Counter