According to Leonardo's preview in the accompanying text, written in mirror writing, it was made as a study of the proportions of the (male) human body as described in Vitruvius:

a palm is the width of four fingers
a foot is the width of four palms
a cubit is the width of six palms
a pace is four cubits
a man's height is four cubits (and thus 24 palms)
"erit eaque mensura ad manas pansas" (Literally: "It will be the same in measure to the spread out hands.")
the length of a man's outspread arms (arm span) is equal to his height
the distance from the hairline to the bottom of the chin is one-tenth of a man's height
the distance from the top of the head to the bottom of the chin is one-eighth of a man's height
the distance from the bottom of the neck to the hairline is one-sixth of a man's height
the maximum width of the shoulders is a quarter of a man's height (one cubit)
the distance from the middle of the chest to the top of the head is a quarter of a man's height (one cubit)
the distance from the elbow to the tip of the hand is a quarter of a man's height (one cubit)
the distance from the elbow to the armpit is one-eighth of a man's height (half a cubit)
the length of the hand is one-tenth of a man's height
the distance from the bottom of the chin to the nose is one-third of the length of the head
the distance from the hairline to the eyebrows is one-third of the length of the face
the length of the ear is one-third of the length of the face
the length of a man's foot is one-sixth of his height

Leonardo is clearly illustrating Vitruvius' De architectura 3.1.2-3 which reads:

For the human body is so designed by nature that the face, from the chin to the top of the forehead and the lowest roots of the hair, is a tenth part of the whole height; the open hand from the wrist to the tip of the middle finger is just the same; the head from the chin to the crown is an eighth, and with the neck and shoulder from the top of the breast to the lowest roots of the hair is a sixth; from the middle of the breast to the summit of the crown is a fourth. If we take the height of the face itself, the distance from the bottom of the chin to the under side of the nostrils is one third of it; the nose from the under side of the nostrils to a line between the eyebrows is the same; from there to the lowest roots of the hair is also a third, comprising the forehead. The length of the foot is one sixth of the height of the body; of the forearm, one fourth; and the breadth of the breast is also one fourth. The other members, too, have their own symmetrical proportions, and it was by employing them that the famous painters and sculptors of antiquity attained to great and endless renown. Similarly, in the members of a temple there ought to be the greatest harmony in the symmetrical relations of the different parts to the general magnitude of the whole. Then again, in the human body the central point is naturally the navel. For if a man be placed flat on his back, with his hands and feet extended, and a pair of compasses centred at his navel, the fingers and toes of his two hands and feet will touch the circumference of a circle described therefrom. And just as the human body yields a circular outline, so too a square figure may be found from it. For if we measure the distance from the soles of the feet to the top of the head, and then apply that measure to the outstretched arms, the breadth will be found to be the same as the height, as in the case of plane surfaces which are perfectly square.[5]

The Ideal body composition is your genetic potential.

http://www.youtube.com/watch?v=2PdJFbjWHEU


Proper Exercise,
and
Proper Nutrition

These two will allow your body to approach what is your genetic potential.
For some that is long slender muscles, for others it will be more bulk.
Regardless of which one you have, you'll approach the one that is encoded in your genes.

You cannot alter your genes (yet) Thus you cannot alter your potential muscle composition, and within short time you should be able to guess which way it's going.

I can bench 200, but that doesn't mean shit, I still have a slim body. (Like a swimmer, but I never swim.... unless to avoid drowning.)
I have a friend who can bench the exact same amount and he's bulky with large upper arms.

The point is; it's genetic.
With proper nutrition and exercise your body will recompose itself into that which is it's optimal state as encoded.
(consider it your state of perfect balance. It's where your body will function to the best of it's ability.)



As an example of the misconceptions at large;
people start swimming in order to get the swimmer's body.

By the same token of logic I should start doing basketball in order to get taller.

It's not the activity that creates the body, it just applies selective pressure (equal to evolution by natural selection, but much much higher competitive pressure.)

brad pitts body in fight club.

Aslong as your shredded thats all that matters. Chicks dont actually look at size, just definition.