SAID principle asserts that the human body adapts specifically to imposed demands. 
In other words, given stressors on the human system, whether biomechanical or neurological, there will be a Specific Adaptation to Imposed Demands (SAID).
For example, by only doing pull-ups on the same regular pull-up bar, your body becomes adapted to this specific physical demand, but not necessarily adapted to other climbing patterns or environments.
The same David Beckham’s legs (and other footballers) are powerful, huge.
However when it comes to their upper torso (although relatively LEAN) are not huge when compared with a bodybuilder.
However, a person’s look is also largely determined by one’s genetics—capacity to build muscle, recovery abilities, skeleton framework and so forth.
The NUMBER ONE THING THAT IS COMMON BETWEEN FOOTBALLERS, BODYBUILDERS, FITNESS MODELS OR ANYONE WITH A GREAT PHYSIQUE IS THAT
- They have low body fat levels, <12%
- They have decent levels of muscle mass
That’s all you need to know.
If you are above, 15% body fat percentage for men, get on a caloric deficit diet and bring it down.
Strive to be in the sexy weight range.
You will love these posts if you want to know the exact body proportions and other things to build an ideal male physique.
Also, many people tend to underestimate their body fat percentages. Learn what range you are in: