Sorry. The issue is with the nutrition, it has nothing to do with the training. Training simply does not increase body fat. Ask for a refund and find a trainer who knows what they are talking about.

Tell that to Jack LaLane, Lee Labrada, Arnold Schwarzenegger, or any other number of people who enjoy a very healthy, high quality of life due in a large part to their weight training background.

Someone with weak shoulders will also suffer in the dumbbell movement, and increase their risk of injury with the barbell. This is why both are actually important in a routine - the dumbbell for the unilateral isolation of various muscles to allow adequate loading/coordination response, and the barbell to increase the loading directly on the chest by taking out some of the synergists and stabilizers as weak links.

You have muscle cells that are grouped in bundles called motor units. They are called units because each of these muscle cells has a single nerve that commands them to contract or relax. Each motor unit is either completely on, or completely off - there is no in-between.

So, when it comes to strength, think of it like tug of war. You have two teams with 200 people each. However, let's say you start out and team A has all 200 pulling at once, while team B only has 50 people. Who do you think will win?

When you are using your muscles, your motor units switch on in groups. More motor units at once means more force generated. This is how you can control speed - while each unit is either on or off, your central nervous system will command groups to switch on at the same time - just enough to allow you to move the weight at that speed.

So, your two people are training differently. One person has perhaps some more genetic potential for mass and/or has trained to gain mass. The other person has more efficient muscles. Your body never effectively utilizes 100% of your motor units. This is why you can find someone small with a lot of strength - their muscles are more efficient. The strongest power-lifters don't have muscles as big as the biggest bodybuilders for this reason. It's a different response and stimulus.

I know a guy who weighs 180 and can bench press over 400 pounds and can do 225 for 20 - 30 reps due to how he trained.

I always want to give credit where credit is due, so I must share that I learned the "weak side rule" from Ian King, author of The Book of Muscle.

The weak side rule is simple:

1. Use unilateral movements. Unilateral means one-sided. If it's your arm, do one-armed curls rather than using both arms. If it's your back, like me, do one-armed rows or one-armed lat pull-downs so you can focus on one side at a time.
2. Always train the weak side first. There are two reasons for this. First, you want to assess the strength of the weakest side so you can adjust the strong side. Second, your first muscle worked is what is prioritized in the workout — it receives the most energy and attention.
3. Never do more with the strong side than the weak side can handle. This is the heart of the rule. Basically, if you can only handle 10 curls at 20 pounds on the weak side, this is ALL you do on the strong side! Work the weak side to failure, if that is your routine, but always use the same weight with the strong side and stop at the same number of reps that your weak side could handle. This allows the weak side to be the side that is stressed the most and grows to "catch up" with the strong side. Don't worry, you won't lose major muscle mass on the strong side — it receives enough stimulation to maintain while the weaker side is catching up!