I would not go as far as saying it is a constant, and i do agree with you, that it has changed less than the womens standard.
But what i do think that throughout history the ideal of a mans body can still always be categorized as lean. It has always been inspired by power in combination with the slenderness of youth. And throughout the ages we find periods were the standard is a little slimmer. And we also are not the first culture to be obsessed with it, in ancient greek philosophy we find a strong appreciation for male beauty which is defined by the young athlete.
Young men worked harder and sacrificed more for their looks than women did.
Nietzsche in Twilight of the Idols claims that the christian contemptuousness is responsible for this not carrying on. He is a big admirer of the ancient greek idea of beauty, as he is of the revival of that standard in renaissance and enlightenment. he thought that tastefulness requires great sacrifices, which also does reflects the current zeitgeist of the 19th century.
So my point is this ideal and obsession with it is not new, rather than it has always been that way. I do acknowledge change and adaption of the standard.
I'm not sure I agree that the standard has always been lean, at least not lean in the sense that it is currently used. Even if for the sake of an example we only look at images from ancient Greece, what we see are several different classes of male physical archetype.
Images of Greek athletes, mostly sculptural, tend to show men with extremely robust musculature, indeed the classical standard in heroic sculpture is famously out of proportion since it shows the head as being far smaller than it should be in comparison to the epic size of the body and limbs. In most cases it's difficult to even describe ancient Greek images of athletes as lithe, and in no way could one describe them as thin. The musculature is bulbous, thick, these are bodies which are well fed, and strong. They don't have excess fat certainly, and they are quite vascular, but they're not strictly speaking lean.
Here is a reasonable modern analogue -
http://lh4.ggpht.com/_c9BPAbXRnys/T...lint-mauro-santiago-sierra-homotography-1.jpg
The guy in that pic is athletic, not lean, has has low body fat and his muscles are well defined but he isn't skinny and his skeletal architecture is well covered.
Images of Greek younger men, Ephebes and the like, are even less lean. In a world where it was sometimes difficult to be certain of a person's exact age, and bearing in mind that ancient Greece was an Age Class dominated society (meaning that different age groups were expected to behave in certain ways and mix mostly only within their age class, and have different kinds of responsibility towards the polis according to their age) it was the physical appearance of signs of age which formed the basis of defining a man's social status.
So while again, the images of Ephebes and youths are often fit, muscular and healthy, in line with the social custom of requiring youths to visit the gymnasion regularly in order to prepare them for the physical requirements of being a man, they are deliberately shown as being softer, fuller, more rounded, less angular and with less muscle definition than older men. Indeed depending on a range of subtle visual signals, such as length of sideburns, position of hair on the body, overall proportions of the musculature it was intended to be possible to be able to tell which age class the youth actually fitted in, the Greeks had several. Very few of these images would be considered lean by the standard of the current fashionable use of that term, since showing some degree of softness and remaining "puppy fat" (even if in only tiny amounts) was necessary to indicate the youthfulness of the young man depicted.
Older men, which is to say men whose contribution to the polis would no longer be expected to include going to war or be of a primarily physical nature come in a slightly more obvious range of physical types.
There are the Old War Heroes, with the super massive heroic musculature of the Athlete/Hero shown but with less torsion and tension. The body is shown as slackened the skin just ever so slightly less tight, in some cases the former perfection is shown as having been marred by the healing of wounds, making the body look somewhat tired even. Again this body isn't lean, because the body is shown as well fed, and bulky, the reward of physical labour and violent exertions on the part of the polis being bounty and feasting.
There are also Statesmen/Philosophers, whose bodies are perhaps the closest to the current standard of leaness which ancient greek figurative art depicts. These older men are shown with the universal musculature, the result of the universal requirement of all citizens to be physically fit, but their bodies are not shown with the super massive muscles of the Athlete/War Hero, instead their bodies are shown as being more lithe, more atenuated, more bony and in some cases even wizened and skinny by ancient Greek standards. Ribs, and other skeletal architecture are more pronounced in this group of images of ancient Greek men than in almost any other. But it is debatable as to whether these images were presented as aspirational archetypes and in city states with little or no real democracy they are infrequent
There are other kinds of image of ancient Greek male ideal, but they occur less frequently.
My point is that even when these images show men who would have a very low body fat percentage they do not show men who are lean in the modern sense of the word, because even when these men are in the prime of physical development they are depicted as having a musculature which is too robust too large and too dense to fit the current modern fashion for leaness.
So I'll certainly agree that the standard of male beauty has frequently tended to been fit, muscular and low in body fat, but not lean. Indeed leaness, in ancient Greece, and in other historical cultures was seen as a physical attribute of the poor and under classes whose bodies were expected to do harsh labour on meager nutrition. Slaves and peasants were shown with good muscle definition, but their bodies were thin and their limbs puny (even if well defined) by comparison with the thick heavy muscles aspired to by the upper classes whose body image was the only image prized in most ancient cultures, especially the classical Greeks. The current Lean fashion in male beauty most closely resembles images of slaves and peasants from ancient art rather than heroes, gods, athletes and Kings.