As a bodybuilder,or budding weightlifter looking for mass, you have no doubt already concluded that there is a definite realtionship between the amount of weight you use in all of your exercises and the size of your muscles.The heavier you train, the larger will be ur muscles.It is possible 2 gain fatty body weight simply by pigging out on junk food several times a day, but real muscle mass is gained by using the proper methods and staying consistant to your training.You should gain weight by increasing muscle mass,which means that u must train with heavy weights and low reps on basic exercises,on large compund movements (SQAUT and the BENCH PRESS) you can use very heavy weights, and these are easily the best exercises for building muscle mass. In order 2 use maximum weight for low reps without the fear of incurring an injury,proper conditioning to get ready for this type of training must b done for begginers.You must thoroghly warm up the muscles called into play with each movement. the best way 2 do this is 2 use a technique call PYRAMIDDING* in which u begin each exercise using a light weight for high reps, then increase the weight and decrease the reps on each suceeding set. Heres a good three day per week weight gaining program:


EXERCISE SETS REPS
1.Sit-ups with added weight 2-3(sets) 25-50(reps)
2.Squat 5 (sets) 12/10/8/6/4*
3.Barbell Bent Rowing 4 (sets) 12/10/8/6/5*
4.Deadlift 3 (sets) 8/6/4*
5.Barbell Shrug 3 (sets) 12/10/8*
6.Bench Press 5 (sets) 12/10/8/6/4
7.Military Press 3 (sets) 8/6/4*
8.Barbell Curl 3 (sets) 8/6/4*
9.Lying tricep Extension 3 (sets) 10/8/6*
10.Barbell Toe Raise 4(sets) 12/10/8/6*
these exercises shud be pyrammidded *

there are many other methods such as stripping(which is rather the opposite of pyrammiding),polarization and many more

I will be posting more bodybuidling/strength training articles and any other topics regarding weightlifting,and am here to help other members with these issues

peace