In terms of natural history of the human body it tells us that an idealised male body looks now pretty much like it did then, when the ancients produced idealised forms we see and even also prefer today – in cultures imprinted with a hellenistic legacy.

An assumed 'level of fitness' or even health is not that well correlated to mere appearances. We still see well formed athletes die today, getting numerous health problems a moderate leisure sportsman avoids. Fat-thin people and tofis ("thin on the outside, fat on the inside", meaning not visibly overweight but still very unfit organs on the verge of being disabled by enveloping fat deposits).

From what we read about the accomplishments of some ancient warriors or athletes it also stands to reason to doubt a that much higher level of general 'fitness' in modern times. We know that fighting in heavy armour under scorching sun is exhausting to the extreme, but we do not know what a probably legendary Pheidippides actually is supposed to have 'done' at Marathon, when the battle was on. Being a message runner or even fighter at the battle would probably have meant a suboptimal starting condition for such a distance. And also in modern times a list of marathon fatalities has its own thoroughly incomplete Wiki page.

What we do learn from such ancient depictions as often presented in museums is a relatively stable preference for the popular aesthetic of male body physique. If your task is to depict a god, it tends to be 'perfect', similar if you depict a 'hero', or a powerful man. Later, if another 'you' come across a 'perfect' statue, the 'beauty' is admired, and kept. 'Ugly statues' are thought to be not that ideal for paying entrance fees.

But the fact to note is that ancient sculptors did not "depict only the strongest physiques". There was a bias for this kind of ideal body, there is a bias now to put these on display in museums. But these preferences also varied with the times, and audiences.

Some artists and some patrons at various points preferred a kind of realism for the art, old age, body fat, bald head, veins and wrinkles. At some points even the grotesque was shown, like giant disfiguring tumors.

Every element and aspect chosen by an artist has its own importance and re ects a conscious choice. Following R.R.R. Smith:

“Images were not reflectors, but like texts and speakers, active participants in public discourse.”

Exaggeration and stress of some features bring to light what was really important for the artist or the client, or the aesthetic appeal of these figurines.

Several ancient depictions depart from the aesthetic ideal widespread in the ancient world. But with the number of those representations, which oscillate between realism and the grotesque, people must have appreciated this kind of art. There is evidence for this in a couple of literary examples:

When we see emaciated people we are distressed, but we took upon statues and paintings of them with pleasure because our minds are captivated by imitations which we find endearing. --Plutarch, Quaestiones Conviviales V 1.

Out of a legacy which I have come in for I have just bought a Corinthian bronze, small it is true, but a charming and sharply-cut piece of work […]. For it is a nude, […]. It represents an old man in a standing posture; the bones, muscles, nerves, veins, and even the wrinkles appear quite life-like; the hair is thin and scanty on the forehead; the brow is broad; the face wizened; the neck thin; the shoulders are bowed; the breast is flat, and the belly hollow. The back too gives the same impression of age, as far as a back view can. […] In fact, in every respect it is a work calculated to catch the eye of a connoisseur and to delight the eye of an amateur, and this is what tempted me to purchase it, although I am the merest novice. --Pliny, Epistulae III 6.

Plutarch and Pliny express clearly the idea of a “double view”: what is considered ugly in everyday life can become a source of admiration in the artistic field. In general, Greco-Roman art displays a standardized beauty resulting from an aesthetic fixed by Polyclitus. Thus, thinness, obesity, old age, or illness were not major artistic themes. Every deviation of every feature from the standard corresponds to an aspect of the character or behavior of the represented subjects.
_{— Sylvain Vanesse: "Between Street Vendors, Singing Slaves, and Envy", Chronika, Vol 6, 2016. (pp15–25)}

So, for assessing any 'level of fitness' for a more general population in former times reliance on artistic representations is one important clue, but a very biased one in terms of creation and transmission through time. Other sources are needed to approximate a more complete picture: for example written accounts, and for people long dead unless they are found frozen or preserved in some other manner: bones.