Why less volume for the smaller muscle groups, you ask? Partially because they are smaller, but mostly because they get a ton of indirect volume while training the bigger muscle groups (e.g. your biceps get hit pretty hard while training back, triceps get hit pretty hard while training chest and shoulders, shoulders get hit pretty hard while training chest, etc.).
Your glutes serve as the main mover and power output to most of your major and accessory lifts—squat, deadlift, kettbell swings, box jumps—so the more you develop and stimulate your gluteal muscles, the more you’re able to maximize control, strength, and stability. (It doesn’t hurt that women love watching you do leg and glute work in the gym, either.)
A flat, atrophied butt doesn't just look bad in jeans or swimsuits—it’s also likely contributing to your tight hips and back problems. If not, it soon will be. That’s because a flat butt is a symptom of tight hips and hamstrings, the result of poor glutes activation. In an ideal world, they all work together to stabilize the pelvis and produce many years of fluid movement.
The high levels of muscle growth and repair achieved by bodybuilders require a specialized diet. Generally speaking, bodybuilders require more calories than the average person of the same weight to provide the protein and energy requirements needed to support their training and increase muscle mass. In preparation of a contest, a sub-maintenance level of food energy is combined with cardiovascular exercise to lose body fat. Proteins, carbohydrates and fats are the three major macronutrients that the human body needs in order to build muscle. The ratios of calories from carbohydrates, proteins, and fats vary depending on the goals of the bodybuilder.
Dynamic stretching (stretching while in motion), is the most efficient way to lengthen and strengthen the hip flexor muscles. An exercise that you can do that will engage the core and all of the lower extremity muscles while focusing on the hip flexors is called the Single Leg Split Squat. Remember that the rectus femoris crosses the hip and knee joint. This exercise involves both the hip and knee joint, making it an appropriate exercise to train the rectus femoris as well as the iliacus.
Note that this recommendation is for total weekly volume, which means it would need to be divided up based on how many times you’re training each muscle group per week. So, for example, someone training everything twice per week would do 30-70 reps for each bigger muscle group in each of those workouts, and 15-35 reps for each smaller muscle group in each of those workouts.