Optimal Pectoral Size Training
There are different strategies for training pectorals depending on if your goal is size, strength, or a combination of both. In general, the muscles in the upper body recover more quickly, meaning that you can/should train those lifts more frequently. The pectorals benefit from exercises that push wider ranges of motion - which is why the barbell bench press isn’t the ideal exercise for increasing pec size. Paused reps, “squeezing,” and slow, deliberate movement are key. Inclines and variations are also important to target all of the pectoral muscle fibers. Be sure to include horizontal presses, vertical presses from below, and the incline fly.
Developing The Delts
The deltoids are an important muscle group that work in conjunction with the back, shoulders, and pectorals. They are a small muscle group and therefore they grow more slowly. Training them is a long term goal that requires patience, strict form, and more patience. The trapezius muscles (traps) are also key in developing the delts and are also difficult to isolate and train individually. There are many excercises that help specifically build up the delts and traps, but they can also be trained during your normal lifting routine. The posterior delts are the least targeted during regular training so if you want to develop the delts, make sure you focus on this area.
Muscle Activation Is More Dependent On Load Than Form
It's commonly believed that small tweaks to your exercise form can sometimes have huge results in terms of muscle activation, and you can use this to preferentially target certain muscles. However, it's far more important to focus on good progression first.