Most people start physical training to alter their body shape. A few will be looking to gain muscle size, or train for performance, but most people want to reduce body size (or more importantly; body fat). Making changes to our body shape is predominantly down to what we eat and drink, in other words, calorie control. Yet the shocking truth is, exercise is an inefficient form of calorie control. Yes, you can be fit & fat! Exercise does drive our bodies to change, but slowly without quality fuel. You may have a killer training session in the gym, but a (insert dietary no-no here!) later on can easily undo all that hard work, and even put in more calories than you burned. You could have a great gym, a personal trainer and be following the best program in the world, but if you neglect to put effort into changing your diet, you are setting yourself up to fail, or at best, driving with the handbrake on.
Remember, we get healthy to lose weight, we do not lose weight to get healthy.